diff --git a/src/__tests__/billing-branch-coverage.test.ts b/src/__tests__/billing-branch-coverage.test.ts
index ad2cac9..ba951c0 100644
--- a/src/__tests__/billing-branch-coverage.test.ts
+++ b/src/__tests__/billing-branch-coverage.test.ts
@@ -257,4 +257,334 @@ describe("Billing Branch Coverage", () => {
       );
     });
   });
+
+  describe("Success route - customerId branch (line 163)", () => {
+    it("should return 400 when session.customer is null (not a string)", async () => {
+      mockStripe.checkout.sessions.retrieve.mockResolvedValue({
+        id: "cs_null_customer",
+        customer: null,  // Explicitly null, not falsy string
+        customer_details: { email: "test@example.com" },
+      });
+
+      const res = await request(app).get("/v1/billing/success?session_id=cs_null_customer");
+      
+      expect(res.status).toBe(400);
+      expect(res.body.error).toContain("No customer found");
+    });
+
+    it("should return 400 when customer is empty string", async () => {
+      mockStripe.checkout.sessions.retrieve.mockResolvedValue({
+        id: "cs_empty_customer",
+        customer: "",  // Empty string is falsy
+        customer_details: { email: "test@example.com" },
+      });
+
+      const res = await request(app).get("/v1/billing/success?session_id=cs_empty_customer");
+      
+      expect(res.status).toBe(400);
+      expect(res.body.error).toContain("No customer found");
+    });
+
+    it("should return 400 when customer is undefined", async () => {
+      mockStripe.checkout.sessions.retrieve.mockResolvedValue({
+        id: "cs_undef_customer",
+        customer: undefined,
+        customer_details: { email: "test@example.com" },
+      });
+
+      const res = await request(app).get("/v1/billing/success?session_id=cs_undef_customer");
+      
+      expect(res.status).toBe(400);
+      expect(res.body.error).toContain("No customer found");
+    });
+  });
+
+  describe("Webhook checkout.session.completed - hasDocfastProduct branch (line 223)", () => {
+    it("should skip webhook event when line_items is undefined", async () => {
+      mockStripe.webhooks.constructEvent.mockReturnValue({
+        type: "checkout.session.completed",
+        data: {
+          object: {
+            id: "cs_no_items",
+            customer: "cus_no_items",
+            customer_details: { email: "test@example.com" },
+          },
+        },
+      });
+
+      // Mock: line_items is undefined
+      mockStripe.checkout.sessions.retrieve.mockResolvedValue({
+        id: "cs_no_items",
+        line_items: undefined,
+      });
+
+      const { createProKey } = await import("../services/keys.js");
+      const res = await request(app)
+        .post("/v1/billing/webhook")
+        .set("content-type", "application/json")
+        .set("stripe-signature", "valid_sig")
+        .send(JSON.stringify({ type: "checkout.session.completed" }));
+
+      expect(res.status).toBe(200);
+      expect(createProKey).not.toHaveBeenCalled();
+    });
+
+    it("should skip webhook event when line_items.data is empty", async () => {
+      mockStripe.webhooks.constructEvent.mockReturnValue({
+        type: "checkout.session.completed",
+        data: {
+          object: {
+            id: "cs_empty_items",
+            customer: "cus_empty_items",
+            customer_details: { email: "test@example.com" },
+          },
+        },
+      });
+
+      mockStripe.checkout.sessions.retrieve.mockResolvedValue({
+        id: "cs_empty_items",
+        line_items: { data: [] },  // Empty array - no items
+      });
+
+      const { createProKey } = await import("../services/keys.js");
+      const res = await request(app)
+        .post("/v1/billing/webhook")
+        .set("content-type", "application/json")
+        .set("stripe-signature", "valid_sig")
+        .send(JSON.stringify({ type: "checkout.session.completed" }));
+
+      expect(res.status).toBe(200);
+      expect(createProKey).not.toHaveBeenCalled();
+    });
+
+    it("should skip webhook event when price is null", async () => {
+      mockStripe.webhooks.constructEvent.mockReturnValue({
+        type: "checkout.session.completed",
+        data: {
+          object: {
+            id: "cs_null_price",
+            customer: "cus_null_price",
+            customer_details: { email: "test@example.com" },
+          },
+        },
+      });
+
+      mockStripe.checkout.sessions.retrieve.mockResolvedValue({
+        id: "cs_null_price",
+        line_items: {
+          data: [{ price: null }],  // Null price
+        },
+      });
+
+      const { createProKey } = await import("../services/keys.js");
+      const res = await request(app)
+        .post("/v1/billing/webhook")
+        .set("content-type", "application/json")
+        .set("stripe-signature", "valid_sig")
+        .send(JSON.stringify({ type: "checkout.session.completed" }));
+
+      expect(res.status).toBe(200);
+      expect(createProKey).not.toHaveBeenCalled();
+    });
+
+    it("should skip webhook event when product is null", async () => {
+      mockStripe.webhooks.constructEvent.mockReturnValue({
+        type: "checkout.session.completed",
+        data: {
+          object: {
+            id: "cs_null_product",
+            customer: "cus_null_product",
+            customer_details: { email: "test@example.com" },
+          },
+        },
+      });
+
+      mockStripe.checkout.sessions.retrieve.mockResolvedValue({
+        id: "cs_null_product",
+        line_items: {
+          data: [{ price: { product: null } }],  // Null product
+        },
+      });
+
+      const { createProKey } = await import("../services/keys.js");
+      const res = await request(app)
+        .post("/v1/billing/webhook")
+        .set("content-type", "application/json")
+        .set("stripe-signature", "valid_sig")
+        .send(JSON.stringify({ type: "checkout.session.completed" }));
+
+      expect(res.status).toBe(200);
+      expect(createProKey).not.toHaveBeenCalled();
+    });
+  });
+
+  describe("Webhook customer.updated event (line 284-303)", () => {
+    it("should sync email when both customerId and newEmail exist", async () => {
+      mockStripe.webhooks.constructEvent.mockReturnValue({
+        type: "customer.updated",
+        data: {
+          object: {
+            id: "cus_email_update",
+            email: "newemail@example.com",
+          },
+        },
+      });
+
+      const { updateEmailByCustomer } = await import("../services/keys.js");
+      vi.mocked(updateEmailByCustomer).mockResolvedValue(true);
+
+      const res = await request(app)
+        .post("/v1/billing/webhook")
+        .set("content-type", "application/json")
+        .set("stripe-signature", "valid_sig")
+        .send(JSON.stringify({ type: "customer.updated" }));
+
+      expect(res.status).toBe(200);
+      expect(updateEmailByCustomer).toHaveBeenCalledWith("cus_email_update", "newemail@example.com");
+    });
+
+    it("should not sync email when customerId is missing", async () => {
+      mockStripe.webhooks.constructEvent.mockReturnValue({
+        type: "customer.updated",
+        data: {
+          object: {
+            id: undefined,  // Missing customerId
+            email: "newemail@example.com",
+          },
+        },
+      });
+
+      const { updateEmailByCustomer } = await import("../services/keys.js");
+
+      const res = await request(app)
+        .post("/v1/billing/webhook")
+        .set("content-type", "application/json")
+        .set("stripe-signature", "valid_sig")
+        .send(JSON.stringify({ type: "customer.updated" }));
+
+      expect(res.status).toBe(200);
+      expect(updateEmailByCustomer).not.toHaveBeenCalled();
+    });
+
+    it("should not sync email when email is missing", async () => {
+      mockStripe.webhooks.constructEvent.mockReturnValue({
+        type: "customer.updated",
+        data: {
+          object: {
+            id: "cus_no_email",
+            email: null,  // Missing email
+          },
+        },
+      });
+
+      const { updateEmailByCustomer } = await import("../services/keys.js");
+
+      const res = await request(app)
+        .post("/v1/billing/webhook")
+        .set("content-type", "application/json")
+        .set("stripe-signature", "valid_sig")
+        .send(JSON.stringify({ type: "customer.updated" }));
+
+      expect(res.status).toBe(200);
+      expect(updateEmailByCustomer).not.toHaveBeenCalled();
+    });
+
+    it("should not sync email when email is undefined", async () => {
+      mockStripe.webhooks.constructEvent.mockReturnValue({
+        type: "customer.updated",
+        data: {
+          object: {
+            id: "cus_no_email_2",
+            email: undefined,  // Undefined email
+          },
+        },
+      });
+
+      const { updateEmailByCustomer } = await import("../services/keys.js");
+
+      const res = await request(app)
+        .post("/v1/billing/webhook")
+        .set("content-type", "application/json")
+        .set("stripe-signature", "valid_sig")
+        .send(JSON.stringify({ type: "customer.updated" }));
+
+      expect(res.status).toBe(200);
+      expect(updateEmailByCustomer).not.toHaveBeenCalled();
+    });
+
+    it("should log when email update returns false", async () => {
+      mockStripe.webhooks.constructEvent.mockReturnValue({
+        type: "customer.updated",
+        data: {
+          object: {
+            id: "cus_no_update",
+            email: "newemail@example.com",
+          },
+        },
+      });
+
+      const { updateEmailByCustomer } = await import("../services/keys.js");
+      vi.mocked(updateEmailByCustomer).mockResolvedValue(false);  // Update returns false
+
+      const res = await request(app)
+        .post("/v1/billing/webhook")
+        .set("content-type", "application/json")
+        .set("stripe-signature", "valid_sig")
+        .send(JSON.stringify({ type: "customer.updated" }));
+
+      expect(res.status).toBe(200);
+      expect(updateEmailByCustomer).toHaveBeenCalledWith("cus_no_update", "newemail@example.com");
+      // The if (updated) branch should not be executed when false
+    });
+  });
+
+  describe("Webhook default case", () => {
+    it("should handle unknown webhook event type", async () => {
+      mockStripe.webhooks.constructEvent.mockReturnValue({
+        type: "unknown.event.type",
+        data: { object: {} },
+      });
+
+      const res = await request(app)
+        .post("/v1/billing/webhook")
+        .set("content-type", "application/json")
+        .set("stripe-signature", "valid_sig")
+        .send(JSON.stringify({ type: "unknown.event.type" }));
+
+      expect(res.status).toBe(200);
+      expect(res.body.received).toBe(true);
+    });
+
+    it("should handle payment_intent.succeeded webhook event", async () => {
+      mockStripe.webhooks.constructEvent.mockReturnValue({
+        type: "payment_intent.succeeded",
+        data: { object: {} },
+      });
+
+      const res = await request(app)
+        .post("/v1/billing/webhook")
+        .set("content-type", "application/json")
+        .set("stripe-signature", "valid_sig")
+        .send(JSON.stringify({ type: "payment_intent.succeeded" }));
+
+      expect(res.status).toBe(200);
+      expect(res.body.received).toBe(true);
+    });
+
+    it("should handle invoice.payment_succeeded webhook event", async () => {
+      mockStripe.webhooks.constructEvent.mockReturnValue({
+        type: "invoice.payment_succeeded",
+        data: { object: {} },
+      });
+
+      const res = await request(app)
+        .post("/v1/billing/webhook")
+        .set("content-type", "application/json")
+        .set("stripe-signature", "valid_sig")
+        .send(JSON.stringify({ type: "invoice.payment_succeeded" }));
+
+      expect(res.status).toBe(200);
+      expect(res.body.received).toBe(true);
+    });
+  });
 });
diff --git a/src/__tests__/demo-branch-coverage.test.ts b/src/__tests__/demo-branch-coverage.test.ts
new file mode 100644
index 0000000..05befe4
--- /dev/null
+++ b/src/__tests__/demo-branch-coverage.test.ts
@@ -0,0 +1,223 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import express from "express";
+import request from "supertest";
+
+vi.mock("../services/browser.js", () => ({
+  renderPdf: vi.fn(),
+  renderUrlPdf: vi.fn(),
+}));
+
+let app: express.Express;
+
+beforeEach(async () => {
+  vi.clearAllMocks();
+  vi.resetModules();
+
+  const { renderPdf } = await import("../services/browser.js");
+  vi.mocked(renderPdf).mockResolvedValue({ pdf: Buffer.from("%PDF-1.4 mock"), durationMs: 10 });
+
+  const { demoRouter } = await import("../routes/demo.js");
+  app = express();
+  app.use(express.json({ limit: "500kb" }));
+  app.use("/v1/demo", demoRouter);
+});
+
+describe("Demo Branch Coverage", () => {
+  describe("injectWatermark fallback branch (line 19)", () => {
+    it("should append watermark when full HTML document doesn't contain </body> tag", async () => {
+      const { renderPdf } = await import("../services/browser.js");
+      // Send full HTML (with <html>) but without </body> to hit the fallback branch
+      const htmlWithoutClosingBody = `
+        <html>
+          <head><title>Test Page</title></head>
+          <body>
+            <h1>Hello</h1>
+            <p>Content here</p>
+      `;
+
+      const res = await request(app)
+        .post("/v1/demo/html")
+        .set("content-type", "application/json")
+        .send({ html: htmlWithoutClosingBody });
+
+      expect(res.status).toBe(200);
+      expect(res.headers["content-type"]).toMatch(/application\/pdf/);
+
+      // Verify watermark was appended (not replaced) 
+      const calledHtml = vi.mocked(renderPdf).mock.calls[0][0];
+      
+      // The fallback should append the watermark at the end
+      expect(calledHtml).toContain("Hello");
+      expect(calledHtml).toContain("Content here");
+      expect(calledHtml).toContain("DEMO");
+      expect(calledHtml).toContain("Generated by DocFast");
+      
+      // Ensure the original HTML is preserved before the watermark
+      expect(calledHtml.indexOf("Hello")).toBeLessThan(calledHtml.indexOf("DEMO"));
+      
+      // Ensure watermark is appended at the end (since there's no </body> to replace)
+      const lastBodyCloseIndex = calledHtml.lastIndexOf("</body>");
+      const watermarkIndex = calledHtml.indexOf("Generated by DocFast");
+      // If there's a </body> at the very end (from wrapping), the watermark should be before it
+      if (lastBodyCloseIndex > -1) {
+        expect(watermarkIndex).toBeLessThan(lastBodyCloseIndex);
+      }
+    });
+
+    it("should append watermark to plain HTML fragment without </body>", async () => {
+      const { renderPdf } = await import("../services/browser.js");
+      const res = await request(app)
+        .post("/v1/demo/html")
+        .set("content-type", "application/json")
+        .send({ html: "<div>Simple fragment</div>" });
+
+      expect(res.status).toBe(200);
+      
+      const calledHtml = vi.mocked(renderPdf).mock.calls[0][0];
+      expect(calledHtml).toContain("<div>Simple fragment</div>");
+      expect(calledHtml).toContain("DEMO");
+      expect(calledHtml).toContain("position:fixed;bottom:0;left:0;right:0;");
+    });
+
+    it("should handle markdown that results in HTML without </body> and injects watermark", async () => {
+      const { renderPdf } = await import("../services/browser.js");
+      const res = await request(app)
+        .post("/v1/demo/markdown")
+        .set("content-type", "application/json")
+        .send({ markdown: "# Just a heading\n\nSome text" });
+
+      expect(res.status).toBe(200);
+      
+      const calledHtml = vi.mocked(renderPdf).mock.calls[0][0];
+      // Should contain watermark
+      expect(calledHtml).toContain("DEMO");
+      expect(calledHtml).toContain("Generated by DocFast");
+      expect(calledHtml).toContain("Upgrade to Pro for clean PDFs");
+    });
+
+    it("should still work correctly when HTML contains </body> (replace branch)", async () => {
+      const { renderPdf } = await import("../services/browser.js");
+      const fullHtml = `
+        <html>
+          <head><title>Test</title></head>
+          <body>
+            <h1>Complete HTML</h1>
+            <p>With closing body tag</p>
+          </body>
+        </html>
+      `;
+
+      const res = await request(app)
+        .post("/v1/demo/html")
+        .set("content-type", "application/json")
+        .send({ html: fullHtml });
+
+      expect(res.status).toBe(200);
+      
+      const calledHtml = vi.mocked(renderPdf).mock.calls[0][0];
+      // When </body> exists, watermark should be injected before it
+      expect(calledHtml).toContain("</body>");
+      expect(calledHtml).toContain("DEMO");
+      
+      // The watermark should be between the content and closing </body>
+      const watermarkIndex = calledHtml.indexOf("Generated by DocFast");
+      const closingBodyIndex = calledHtml.indexOf("</body>");
+      expect(watermarkIndex).toBeGreaterThan(-1);
+      expect(closingBodyIndex).toBeGreaterThan(-1);
+      expect(watermarkIndex).toBeLessThan(closingBodyIndex);
+    });
+
+    it("should reject empty HTML input with 400 error", async () => {
+      const res = await request(app)
+        .post("/v1/demo/html")
+        .set("content-type", "application/json")
+        .send({ html: "" });
+
+      // Empty HTML is rejected by validation
+      expect(res.status).toBe(400);
+      expect(res.body.error).toContain("html");
+    });
+
+    it("should handle HTML with multiple </body> tags (uses first)</body>", async () => {
+      const { renderPdf } = await import("../services/browser.js");
+      const htmlWithMultipleBodies = `
+        <html>
+          <body>First body</body>
+          <body>Second body</body>
+        </html>
+      `;
+
+      const res = await request(app)
+        .post("/v1/demo/html")
+        .set("content-type", "application/json")
+        .send({ html: htmlWithMultipleBodies });
+
+      expect(res.status).toBe(200);
+      
+      const calledHtml = vi.mocked(renderPdf).mock.calls[0][0];
+      // replace only replaces the first occurrence
+      expect(calledHtml).toContain("First body");
+      expect(calledHtml).toContain("DEMO");
+      expect(calledHtml).toContain("</body>");
+    });
+  });
+
+  describe("Watermark content verification", () => {
+    it("should include demo watermark with exact styling", async () => {
+      const { renderPdf } = await import("../services/browser.js");
+      const res = await request(app)
+        .post("/v1/demo/html")
+        .set("content-type", "application/json")
+        .send({ html: "<h1>Test</h1>" });
+
+      expect(res.status).toBe(200);
+      
+      const calledHtml = vi.mocked(renderPdf).mock.calls[0][0];
+      // Verify watermark styling
+      expect(calledHtml).toContain("background:rgba(52,211,153,0.92);color:#0b0d11");
+      expect(calledHtml).toContain("z-index:999999");
+      expect(calledHtml).toContain("pointer-events:none");
+    });
+
+    it("should preserve user CSS when injecting watermark", async () => {
+      const { renderPdf } = await import("../services/browser.js");
+      const customCss = "body { background: blue; }";
+      
+      const res = await request(app)
+        .post("/v1/demo/html")
+        .set("content-type", "application/json")
+        .send({ html: "<h1>Test</h1>", css: customCss });
+
+      expect(res.status).toBe(200);
+      
+      const calledHtml = vi.mocked(renderPdf).mock.calls[0][0];
+      // Both watermark and user CSS should be present
+      expect(calledHtml).toContain("DEMO");
+      expect(calledHtml).toContain("background: blue");
+    });
+  });
+
+  describe("Branch coverage for attachment headers", () => {
+    it("should set Content-Disposition to attachment for HTML", async () => {
+      const res = await request(app)
+        .post("/v1/demo/html")
+        .set("content-type", "application/json")
+        .send({ html: "<h1>Hello</h1>" });
+
+      expect(res.status).toBe(200);
+      expect(res.headers["content-disposition"]).toMatch(/^attachment/);
+      expect(res.headers["content-disposition"]).toMatch(/filename="demo\.pdf"/);
+    });
+
+    it("should set Content-Disposition to attachment for markdown", async () => {
+      const res = await request(app)
+        .post("/v1/demo/markdown")
+        .set("content-type", "application/json")
+        .send({ markdown: "# Hello" });
+
+      expect(res.status).toBe(200);
+      expect(res.headers["content-disposition"]).toMatch(/^attachment/);
+      expect(res.headers["content-disposition"]).toMatch(/filename="demo\.pdf"/);
+    });
+  });
+});