Is there a way a Paid In Full stamp can be added to the receipts I send to customers…
They keep emailing me back and asking this, I then go back to the invoice and resend that to them as this has the Paid In Full stamp.
You can possibly avoid issuing the receipt but send your customer the sales invoice or customer statement when you receive the payment.
You can add a custom field to receipts with a note Paid in Full but it doesn’t make sense because the sales invoice is what is paid in full and the receipt is the transaction that resulted in that outcome.
You could also add the stamp with a custom theme. But, as @tony said, it would not make sense, because the receipt is not what is paid in full but rather was causes the sales invoice to be paid in full.