I can share why the balance payable field and PAID watermark appear on your invoices.
The balance field is default on a sales form and QuickBooks will immediately stamp an invoice once paid. To remove the watermark, you'll want to clear the payment.
Let me show you how it's done:
Open the invoice.
Click the payment link below the PaymentStatus.
Click the date when the payment is applied.
In the ReceivePayment window, click the ClearPayment button.
Click Save and close.
I'm adding these resources to learn how to record and send invoices to customers: