feat: add more setup instructions to the readme

Author: samblacklock

examples/spa-elastic-path-payments/README.md

@@ -22,7 +22,64 @@ Before running this example, you'll need:
 
 1. **Elastic Path Commerce Cloud Store** with Elastic Path Payments enabled
 2. **Stripe Account** - Elastic Path Payments is powered by Stripe
-3. **Stripe Publishable Key** - Get this from your Stripe Dashboard
+3. **Stripe Publishable Key** and **Account ID** - Get these from your Stripe Dashboard
+
+---
+
+## Store Setup Requirements
+
+To use this example, your **Elastic Path Commerce Cloud store** must be properly configured:
+
+### 🔌 **Payment Gateway Configuration**
+
+1. **Enable Elastic Path Payments** in Commerce Manager:
+
+   - Go to **Settings** → **Payment Gateways**
+   - Configure **Elastic Path Payments** gateway
+   - Add your **Stripe Connect account**
+   - Enable the gateway and test mode
+
+### 📦 **Product Setup**
+
+Your store needs products in a published catalog for the payment demo to work. Learn more about how to publish a catalog [in the docs](https://elasticpath.dev/docs/commerce-manager/product-experience-manager/catalogs/catalog-configuration):
+
+1. **Product Requirements**:
+
+   - Add at least one **simple product** (not a base product requiring variations)
+   - Products should have **prices** associated with them
+   - Products must be **active** and included in the published catalog
+
+2. **Inventory Configuration** (Optional):
+   - If using inventory management, ensure products have **stock levels** configured
+
+### 🔑 **API Access Setup**
+
+1. **Client Credentials**:
+
+   - Go to **Settings** → **API Keys** and create a new application key
+   - Note your **Client ID** and **Store URL** and use them in your .env
+
+2. **Stripe Keys**:
+   - From your **connected Stripe account**, obtain:
+     - **Publishable Key** (`pk_test_...` for test mode or `pk_live_...` for live)
+     - **Account ID** (`acct_...`) - This identifies your specific Stripe Connect account
+
+### 🚨 **Common Setup Issues**
+
+**"No such payment_intent" Error**:
+
+- **Cause**: Frontend Stripe keys don't match the backend Stripe account
+- **Solution**: Ensure `VITE_STRIPE_PUBLISHABLE_KEY` and `VITE_STRIPE_ACCOUNT_ID` are from the same Stripe account connected to Elastic Path
+
+**"No products available" Error**:
+
+- **Cause**: No products in published catalog, catalog not published, or all products are base products
+- **Solution**: Create and publish a catalog with at least one simple product
+
+**404 Inventory Errors**:
+
+- **Cause**: Product inventory not configured
+- **Solution**: Either configure inventory or ignore (example handles gracefully)
 
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