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