With the RSForm!Pro - PayPal plugin you can add products to the form.
The integration uses the standard PayPal website payment method, the customer will be redirected to the Paypal page when the form is submitted where he will need to log in to his account and complete the transaction.
After the payment is approved, PayPal will send a notification to your site and the submission status will be changed to paid.
If the transaction is not approved within 12 hours the payment will be marked as declined.
This RSForm!Pro plugin is free to download once you have an active RSForm!Pro license.