+1  A: 

You might look into Satchmo's source code. Satchmo is an open source e-commerce app for Django, and I'm pretty sure that it has support for a variety of payment gateways.

Their payment modules appear to work with at least PayPal and Authorize.Net from the list you gave, among others.

Will McCutchen  2010-02-19 16:35:26
The satchmo payment processing code is also being broken out into its own project. http://bitbucket.org/bkroeze/django-bursar/
istruble  2010-02-19 19:32:16

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