question

Sorry, this is unfortunately still in our backlog at the moment (though we did add a specific message for exceeding limits). We'll be adding this as soon as we can.

My apologies, but that's still on our backlog. But that said, the error is specifically "invalid receipt", as stated. What that means is that the client submitted a receipt in the call which either was malformed or simply wasn't for your title. The latter is the most common - there's a readily available hack for iOS and Android devices which swaps the receipt for an old one from "Cut the Rope 2". Titles that validate the receipt directly with Google/Apple are commonly tricked by this, as all they're asking is "is this receipt valid". Which it is. It just happens to be several years old, and not for the title in question. For titles that don't use a secure server-to-server process to validate the receipt and control the inventory, it's unsurprising to find that a significant percentage of their "purchases" are bogus because of this.

The Analytics->Reports tab is where you can find the API Usage report for your title, though - you can see the error rate in it for each API call, accumulated by the hour.