I've never done anything like this, so caveat libror . . .

My thought would be to have a regular form text input for the donation amount. Handle it on a page that has "method of payment" choices that link to https sites, and put a query at the top of the page that sends the amount (and possibly other info) to a database.

This would give you an unofficial shadow database, but the average should be pretty close, even given the occasional mistake.

It would be nice if you could manage to put the amount through to PayPal or whatever, but I don't know how to do it.