IAP purchase callback fail what to do?

So if we get a failed purchase callback how are we supposed to know if the person was charged? Because we assumed if it was a failed callback that meant that the person was not charged. However we did get a failed callback and then got contacted by the person saying they didn't get the item (we handle giving the item in the successful callback)

They were able to provide a receipt showing that they were charged. So how are we supposed to know if a failed callback actually charged a person? I'm assuming if the card is declined it would be a failed callback as well?