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.