payment processors

Currently Rencontre allows you much choice as regards a payment processor and it is well-designed.

But maybe in the future we might want to do our own thing and make our own payment web page outside of what is currently supposed.

Please can you tell me what is the URL / web-page address of the final page which is used to indicate that a successful payment has been made and what HTTP GET or POST key/value pairs need to be specified in order to indicate what has been purchased and for what user? Such information will help ensure this useful piece of software is resilent to any changes in the future.

Many thanks for creating such as useful tool.