That basic XML could be inferred from your OP so it doesn't really help. I am guessing, but only guessing, that you have to have a CGI or a client/server/listener set up at a post-back URL. I don't even know if that's right but if it is, you will have write either a CGI or some kind of listening program. The first might be somewhat easy, the second probably not but maybe so with POE. Anyway, the short version is, your problem isn't clear enough to me (or anyone else it seems) to offer you any code pointers. The postback bit sounds like a security feature/method to me where an exchange to conduct the transaction happens between two apps. A casual Google perusal suggests it might just be the destination URL for the client, whatever page you want them to land on, after the transaction.

Surely you have a specification or documentation for whatever project/package you're using. I suggest bearing down on it or trying Google for more.