robee

The HMAC library in Python still uses MD5 (http://docs.python.org/library/hmac.html) which is known to be exploitable with hash collisions. My opinion is that its not too difficult to roll a MAC setup using some SHA based hash.

Willet payments (http://getwillet.com ) is a similar product but has what I believe a better integration process and lower barriers to entry for both buyers and sellers. Willet supports in-app payments for one time purchases, subscriptions and repeatable purchases.

(Full disclosure: I am a co-founder of Willet )