I underpaid my invoice. What should I do?
If you underpaid, the Bitcoin invoice will not switch into confirming/paid state, but will update itself. In the updated invoice you will see the amount of cryptocurrency that you sent, as well as the amount that is still left to pay. Simply make the second transaction to cover the underpaid amount before the invoice expires.
Once you make sufficient payment, your transaction will be confirmed. You will see a confirmation window with your Transaction IDs on the invoice.