On the merchant I had to process with all they provided was a short ASP code example. No classes, no scripts, just a reference of XML fields, approval code definitions, etc.
Its 100% merchant specific, because there is no real standard of how credit card processing communication occurs, so long as it is ‘secure’ (PCI Compliant). I think some processors even have cloud SOAP services and javabeans that accept credit card data, and some allow raw socket connections and stream communication for integration with lower level programming languages.
So its basically an issue of getting whatever you can from the merchant and piecing together their unique puzzle, and deciding what features you want to implement. I for instance never implemented returns processing, because I preferred to just do it directly through the merchant gateway they provided.