Hey there,
I'm currently working on implementing Facebook Store into one of our games. I can go through the Facebook Canvas payments fine up to the point of confirming the users purchase. Once there, I make the call to ConfirmPurchase and it seems to always return "400 Bad Request PurchaseDoesNotExist" even though I can see the purchase inside the users profile and that it has "Succeeded".
Testing Confirm Purchase here also yields the same results.