I think eCart uses the V1 First Data API.
can you send a link the V2 API documentation you are refering to so we can look into it please.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.