Most processors seem to offer some kind of mode like this, where you send users to a page hosted on their server to handle financial transactions. I'm sure that modules to help with it would be welcome. However, I wonder if there's enough standardization in their integration approach to make a common API possible. If you find that there is, go for it. If not, you could still put them up, but maybe in different namespaces (e.g. PayPal:: and Verisign::).