Thanks Ryan,

That's basically what I was looking for, although I'm using a contrib payment module for myVirtualMerchant, which is bare bones (in fact, I've extended it myself and will post that when I've finished testing things out).

I also discovered the cartlink addon, which is very cool and useful here.

I guess I don't care if I totally bypass the checkout form itself, as long as I can pre-populate the fields there so the user doesn't have to re-enter info. They could just verify and follow the rest of the process. Is there anything existing to allow that (besides just hook_form_alter-ing the form)?

-Roger