David,
I am looking into the St. George one at the moment. Not the API but the Hosted Payment Page (HPP) or the Simple Hosted Payment Page (SHPP) for use with merchant accounts.
The flow is very similar to the way Paypal works.
i.e.
Build URL and direct to St. George with basic info
Shopper completes details
On payment completion, user comes back to site with basic details of payment (tx #, amount, merchant id)
Transaction sent to St. George URL for verification
Database updated to reflect payment
I am hoping it may be not too difficult to implement, but then again, I am no coder.
If anyone would be willing to collaborate on this one with me, please let me know as I could do with some good help and guidance.