I can share some information about adding a Paid watermark on your sales receipts.
Being able to add this option to your sales receipts is currently unavailable in Quickbooks Desktop. Sales receipts, by its nature, are Cash Basis transactions that are deemed paid the moment it is recorded/created.
As a workaround, you can add a paid note on the receipt for your customers to view once you send the transaction. Let me show you how:
Open a sales receipt transaction.
On the CustomerMessage box, type Paid or a short description of the transaction status.
Click Save & Close.
Once done, resend the transaction to your customers. They should be able to view the message added to that field.
I can see how helpful this feature can do to your business so I'd recommend sending your product preference to our engineers. They are the ones who consider feedback for QuickBooks enhancements. Let me guide you on how to do that: