Software developer requires on-line service to manage unit..

I have an application in mind that requires to know if an individual
is in credit or not. Each person using my system would need to put
themselves in credit using a top-up swipe card or mobile phone or via
web access or some other method.

Now i dont want to get into all the financial stuff with all the
necessary security etc, so all I want is a 3rd party service that
offers:

{a) secure methods for inviduals to manage their credit (they would
purchase units)
(b) access for my software to get back account status
(c) access for my software to use up credits
(d) probably some other management type access as well, to setp new
accounts etc


Has anyone any ideas ?

Thank you.