Commerce DPS integrates DPS (aka PaymentExpress) with Drupal Commerce payment and checkout system.
Commerce DPS supports both merchant and gateway hosted billing methods, and token billing (in progress).
PX Pay is a payment solution hosted by Payment Express; your site won't see credit card details so you have less requirements for compliance. It's safer and simpler to implement.
PX Post is a payment solution you host on your site. Credit cards are entered at checkout on your site. You need to check with your bank whether they permit merchant-hosted checkout.
Pretty much all the details required in the configuration interface are obtained from Payment Express. If the inline documentation isn't clear, please file an issue and help improve the interface! See below for instructions.
For any questions, the first point of reference should be the issue queue on Drupal.org. Please use the search interface provided to see if your problem has been fixed or is currently being worked on, and if not create a new issue to discuss it.
Always check your site error logs (both webserver and Drupal logs) and give detail when making a report.
Documentation fixes are welcome!
We prefer that you use the Drupal.org issue queue, but contributions are welcome via Github also.