For details about integrating with Paypal, see the following links...
Paypal Standard
READ FIRST: Paypal Standar Integration Summary
Doc: Using DAP-Generated Paypal Buy Buttons
Doc: Using Paypal-Hosted Buy Buttons
Doc: DAP Shopping Cart & 1-Click Upsells with Paypal Standard
Paypal Payments Pro
Doc: DAP Shopping Cart & Upsells with Paypal Payments Pro