I set up a Notes item with a $0.00 price and a Suspense (99999 - Intuit calls this Ask My Accountant) General Ledger account.  Since the items are always $0.00, there's no dollar impact on the invoice.  The line item gives me an effective text box on any invoice, purchase order, or bill.  You could create multiple notes with descriptive item names if you have recurring notes, but this workaround is generally for unique notes.  The item will show up on the invoice with the $0.00 amount, but I don't think I've ever heard from a customer with a question about that part.