How to mark printed invoice as paid

I'm assuming that you're using QuickBooks 2009 for Mac. This is what the Help documentation says.

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PRINTING A STATUS STAMP ON A FORM

You can print a status stamp on invoices, sales receipts, credit memos, and purchase orders.

• PENDING: this stamp appears on invoices, sales receipts, and credit memos that you mark as pending.

• PAID: this stamp appears on invoices for which the balance due is zero.

• RECEIVED IN FULL: this stamp appears on purchase orders for which the balance due is zero.

• CLOSED: this stamp appears on purchase orders that you mark as closed.

To print a status stamp:

1. Choose Customers > Create Invoices, Enter Sales Receipts, or Create Credit Memos/Refunds, or Vendors > Create Purchase Orders.

2. From the Template pop-up menu, choose the template for which you'd like to add a status stamp.

3. Click Customize. The Layout Designer opens. 


4. In Layout Designer, choose File > Print Status Stamp, and then choose None, Large, or Small. The status stamp appears in blue when you print the form. 
Note: These settings apply only to the printed status stamp; you can't change their appearance or turn them off onscreen. 


5. Choose File > Save to save the template.

6. To print the form, go back to QuickBooks, open the form's window, and then press P.