If you have already looked into this (and decided to write your own), it would be great to see a blog post or similar on the current state of payments in Python (and what you intend to do differently).
It seems like there's Satchmo, various Satchmo forks, and GetPaid, but I can't tell which are directly or indirectly tied to various frameworks (Django, Plone). Are people just coding directly to payment APIs? Is everyone running on Django or Plone?
http://www.satchmoproject.com/
http://bitbucket.org/bkroeze/django-bursar