I'm looking for an IVR script/ program, hopefully opensource, that I could use to interact with a payment gateway (using XML)?

ht check out VoiceXML (http://www.voicexml.org/) or VoiceXML Development Guide (http://www.vxml.org/). I'll try to find the one company thru all my emails that one client is using also.

You might look into voxeo.com. I used them years ago to build an app somewhat similar to what you mention. They have the servers that hook into the phone networks and can take calls and process your call xml and voice xml orders. Then you only need to build an app that interfaces with both the payment gateway of your choice and the voicexml gateway(voxeo).

Unfortunately, I don't know of any "script" that is either open source or that you can purchase that would do this out of the box...