Perhaps another hint/DP to the Invalid Merchant error:

Yesterday, got the error, and I said, "Oh, did someone load already this morning?" And the friendly/regular manager said, "No, but someone just bought a $500 gift card."

So, my hypothesis is that any loading, whether in or out, counts against the $500/hr store limit. It's a working theory, but now, when I get this error, I will ask not only, "Did someone load before me?" if the answer is "no", I will follow up with, "Did someone just buy a gift card?"