Hello Google team.
First at all i think API it too complicated and little bit stupid. Please take a look at PayPal API for make mobile payments for references.
Situation 1:
1 Player has 2 different devices with one google account and make purchase on one of them. Did google notice second device about purchase?
2 Player make purchase and lost Internet connection right after that. How application or server will know about that?
Here with list of MUST have features for app billing:
1 Server call back from google processing - see best practices from ANY credit processing gates
2 Create refetence between payment and order Id or any other related info. Currently API allow you send description but no way to take it back from google page for processing.
Many players send me mails from google checkout with help request but i cant connect them with game profiles.
Guys before create own bicycles look around maybe somebody make it already better?