diff --git a/packages/join-flow/scss/theme.scss b/packages/join-flow/scss/theme.scss
index 2bc29efe..2664d4bf 100644
--- a/packages/join-flow/scss/theme.scss
+++ b/packages/join-flow/scss/theme.scss
@@ -8,10 +8,10 @@
 }
 
 $black: var(--wp--preset--color--black, #212529);
-$gray: var(--ck-join-form-gray-color, --wp--preset--color--cyan-bluish-gray, #dfdcda);
-$background: var(--ck-join-form-background-color, --wp--preset--color--base, #f4f1ee);
+$gray: var(--ck-join-form-gray-color, var(--wp--preset--color--cyan-bluish-gray, #dfdcda));
+$background: var(--ck-join-form-background-color, var(--wp--preset--color--base, #f4f1ee));
 
-$primary: var(--ck-join-form-primary-color, --wp--preset--color--accent, #007bff);
+$primary: var(--ck-join-form-primary-color, var(--wp--preset--color--accent, #007bff));
 $primary-focus: native-rgb(from $primary r g b, 0.6);
 $danger: var(--wp--preset--color--accent-5, #e3220c);
 
diff --git a/packages/join-flow/src/components/atoms.tsx b/packages/join-flow/src/components/atoms.tsx
index 94a62d83..e58ee1b9 100644
--- a/packages/join-flow/src/components/atoms.tsx
+++ b/packages/join-flow/src/components/atoms.tsx
@@ -142,6 +142,14 @@ export const PlanRadioPanel: FC<PlanRadioPanelProps> = ({
           });
         };
 
+        // Typing a custom amount must select this tier, otherwise the amount
+        // is ignored on submit and the default plan price is charged.
+        const onChangeCustomAmount = () => {
+          if (!checked) {
+            onChange(currentPlan.value);
+          }
+        };
+
         return (
           <Form.Label
             className={
@@ -179,6 +187,7 @@ export const PlanRadioPanel: FC<PlanRadioPanelProps> = ({
                           min={currentPlan.amount || 0.01}
                           step="0.01"
                           max="1000"
+                          onChange={onChangeCustomAmount}
                         />
                       </FormItem>
                     </div>
@@ -231,6 +240,7 @@ export const PlanRadioPanel: FC<PlanRadioPanelProps> = ({
                             min={currentPlan.amount || 0.01}
                             step="0.01"
                             max="1000"
+                            onChange={onChangeCustomAmount}
                           />
                         </FormItem>
                       </div>
diff --git a/packages/join-flow/src/pages/plan.page.test.tsx b/packages/join-flow/src/pages/plan.page.test.tsx
new file mode 100644
index 00000000..b155ac23
--- /dev/null
+++ b/packages/join-flow/src/pages/plan.page.test.tsx
@@ -0,0 +1,59 @@
+import React from 'react';
+import { render, screen, fireEvent, waitFor } from '@testing-library/react';
+import '@testing-library/jest-dom';
+import { PlanPage } from './plan.page';
+
+jest.mock('../env', () => ({
+  get: jest.fn(),
+  getStr: jest.fn(() => ''),
+}));
+
+jest.mock('../components/summary', () => ({
+  Summary: () => null,
+}));
+
+import { get as getEnv } from '../env';
+const mockGetEnv = getEnv as jest.Mock;
+
+const mockOnCompleted = jest.fn();
+
+const MOCK_PLANS = [
+  { value: 'higher', label: 'Higher income', description: '', amount: '20', currency: 'GBP', frequency: 'monthly', allowCustomAmount: false },
+  { value: 'lower', label: 'Lower income', description: '', amount: '5', currency: 'GBP', frequency: 'monthly', allowCustomAmount: false },
+  { value: 'other', label: 'Other', description: '', amount: '1', currency: 'GBP', frequency: 'monthly', allowCustomAmount: true },
+];
+
+beforeEach(() => {
+  mockGetEnv.mockImplementation((key: string) => {
+    if (key === 'MEMBERSHIP_PLANS') return MOCK_PLANS;
+    return false;
+  });
+  mockOnCompleted.mockClear();
+});
+
+describe('PlanPage — custom amount', () => {
+  test('typing a custom amount selects that tier instead of the default', async () => {
+    render(<PlanPage data={{ membership: 'higher' } as any} onCompleted={mockOnCompleted} />);
+
+    const customInput = document.getElementById('other-amount') as HTMLInputElement;
+    expect(customInput).toBeInTheDocument();
+
+    fireEvent.change(customInput, { target: { value: '7' } });
+
+    fireEvent.submit(screen.getByRole('button', { name: /continue/i }).closest('form')!);
+
+    await waitFor(() => expect(mockOnCompleted).toHaveBeenCalled());
+    const submitted = mockOnCompleted.mock.calls[0][0];
+    expect(submitted.membership).toBe('other');
+    expect(Number(submitted.customMembershipAmount)).toBe(7);
+  });
+
+  test('submitting without touching the custom amount keeps the default tier', async () => {
+    render(<PlanPage data={{ membership: 'higher' } as any} onCompleted={mockOnCompleted} />);
+
+    fireEvent.submit(screen.getByRole('button', { name: /continue/i }).closest('form')!);
+
+    await waitFor(() => expect(mockOnCompleted).toHaveBeenCalled());
+    expect(mockOnCompleted.mock.calls[0][0].membership).toBe('higher');
+  });
+});
diff --git a/packages/join-flow/webpack/dev.js b/packages/join-flow/webpack/dev.js
index 79f528e2..fa6feee0 100644
--- a/packages/join-flow/webpack/dev.js
+++ b/packages/join-flow/webpack/dev.js
@@ -19,6 +19,17 @@ module.exports = merge(commonConfig, {
       "Access-Control-Allow-Origin": "*",
       "Access-Control-Allow-Methods": "GET, POST, PUT, DELETE, PATCH, OPTIONS",
       "Access-Control-Allow-Headers": "X-Requested-With, content-type, Authorization"
+    },
+    // The standalone harness has no WordPress backend, so stub the REST
+    // endpoints the form posts to (mirrors mockRestEndpoints in join-e2e).
+    // Bodies are logged so the payload can be inspected in the terminal.
+    setupMiddlewares: (middlewares, devServer) => {
+      devServer.app.use(require("express").json());
+      devServer.app.post("/join/v1/:resource", (req, res) => {
+        console.log(`[mock] POST /join/v1/${req.params.resource}`, JSON.stringify(req.body));
+        res.json({});
+      });
+      return middlewares;
     }
   },
   devtool: "cheap-module-source-map",