2CheckOut is a well know payment gateway and used world wide .. Not like the PayPal or Authorize.net as they do not support all countries.
It supports 3rd party integration, so you just need to have a look on it and integrate it with user-manager.
Check this page for more info: http://www.2checkout.com/community/blog/tag/parameters/
I think that 2CheckOut is Authorize.net & PayPal reseller while it is the best solution to use for any country worldwide. So, you can use to process CreditCard and Paypal payments using the same account and it supports elecronic check and PayPal pay later service
Another idea is to add customized values, so we can add any payment gateway some billing systems can do (http://www.modernbill.com having many ideas could be shared with the user-maneger).
I’ve added a vote to the wiki’s requested features for the 2CheckOut, I hope to find other interested people voting for it also.
Before I forget, the 2CheckOut website is: http://www.2checkout.com or http://www.2co.com
Regards.