I use both Paypal (Premier) and Google Checkout. Both ping e-junkie when a transaction is completed. e-junkie then does two HTTP posts to me -- one to grab the Registration Key and one confirming the details of their transaction.

I don't use any plugins/gems to handle this. e-junkie's documentation is good and it is mostly a matter of copying parameters they pass into the right models for my home-grown sales tracking, then calling sale.save. This takes all of 77 lines, counting whitespace, and about 60 of them are assignment statements.