I have an unusual requirement.  My clients use a dated client/server 
application that runs locally (including the data source).  We have added 
internet functionality to small pieces of the application.  This was done 
with COM objects (DLL's) that are called using from ASP using late-binding 
and automation. 

I am re-writing these DLL's with .NET Web services.  The client server 
application is also being re-written using .net windows forms.  Here is 
the challenge:  the clients would like to add real time credit card 
processing from the client server application as well as the internet 
piece.  My client platforms range from Unix to Windows. 

I am new to internet credit card processing and would appreciate any 
feedback on where to start, good design practices, etc.  My current idea 
is to expose the credit card processing via a .NET web service that all of 
the clients, browser or windows forms, could call. Not knowing a lot about 
on-line credit card processing, I really do not knwo where to start.