How do I integrate PayPal Payments Standard with the WHMCS cart?

Here's how:

1. Log in to your WHMCS admin panel.
2. Select Setup > Payment Gateways.
3. Select All Payment Gateways > PayPal.
4. Check Show on Order Form.
5. Enter a Display name of PayPal.
6. Enter your PayPal Email address in the PayPal Email field.
7. Determine whether you're selling a one-time product or a recurring product, and choose the settings accordingly.
   Note: If the product is a subscription, the invoice includes a recurring product, and the due date is the current date or an earlier date, a PayPal Subscribe button will appear on the client's invoice.
8. Enter your PayPal API Signature.
   Note: API credentials are required if using refunds.
9. Select Currency Conversion if applicable.
10. Review any additional options to determine if they apply to your integration.
11. Click Save Changes.

See also:
WHMCS PayPal Setup Guide