We have a customer where there was a problem on a her plumbing job. We paid to have the shower redone. she is still sending us invoices for supplies that she says we owe. I am willing to pay her these final invoices but I want to send a letter with the payment saying something like: By cashing this check you release Manwill Plumbing from all further liability ........ Can you help me with a letter like that? I just want her to know we have paid for our mistake and we will no longer be paying her anything else.