I don't think the Alacart from Alabanza would be much use.
The version with cc processing is way too expensive.
We currently use Akopia Interchange but there have been a few bugs and its not very easy to customise.
If I were you I would be looking at something highly configurable like commerce.cgi.
Go to http://www.charity-shop.com
for an example.
You could make a version customised to suit your service and then implement it on all accounts you set up.
It interfaces with a lot of CC processors.
You can also make commission on referring people to these services as well.