PayPal's Invoicing service lets you easily send professional business invoices via email. You can:
- Send an invoice for goods or services
- Provide detailed line items by specifying quantity, unit price, item description, shipping details, tax, and currency
- Generate automatically calculated totals
- Simply fill out the invoice, review, and send
- Save up to 10 invoices as templates for future use
- Use PayPal's Invoicing Templates to prefill invoice fields
Currently it does not (as far as I can tell) support recurring invoices, which is the only drawback that I can see at this point.