Mark,
OpenACS has the Payment service contract and two implementations of this contract:
- the Authorize.Net gateway which interfaces to Authorize.net and
- the PayFlowPro gateway which interfaces to Verisign's PayFlowPro.
I don't know what the status of Verisign's PayFlowPro is these days, but my personal experience with Authorize.Net has been excellent. Check their website for availability outside the US though.
/Bart