Ask your client for the Invoice ID URL, which looks like the following example: https://bitpay.com/invoice?id=QCCcVp8Vk3FCAP57w79t5m
When visiting the invoice page, you will be able to see the status of the invoice:
If the invoice is expired, the transaction didn't get applied to the invoice.
If the invoice is underpaid, the invoice has not been credited to the merchant's account.
If the invoice is overpaid, the overpaid amount needs to be claimed by either the purchaser or the merchant.