LowFat Invoice 3.02
the author of this app says:
This program allows you to create invoices, estimates, sales receipts, sales orders, payments, and credit memos, and print or e-mail them directly from your PocketPC. You can zoom in and out, change text sizes and use the built-in line item database that comes with sample items, discounts, subtotals, taxes, customer messages and payment terms.
Add a single item to your invoice or select a group of items to add all at once. If you make a mistake, just use the multi-level undo and redo commands. The program even works with your existing PocketPC contacts.