Forum Moderators: buckworks
Many payment gateways offer a Name-Value-Pair (NVP) interface, which is more of a RESTful interface than SOAP. There are a few (namely PayPal) that offer both a NVP and SOAP interface, and it's personal preference which one to use.
Generally, they return an XML structure, so you want to become familiar with XML Schemas (.xsd files) and the XML (de-)Serializers in the .NET Framework, in order to parse the response the ".NET way".