Possible additions for registered users:
- keep all company info entered for my next visit (maybe it does, did not check)
- every invoice needs a unique incremental number so a list of previous created invoices by number would be useful - also for ease of use
- registration should still be just an option - giving extra features
Expand your market:
- other languages - because its not very nice to send my clients an invoice in English
- VAT (in Europe normally added on a separate line below the total and then another final total)
- 'make all checks payable' = interesting, in Europe we normally transfer by bank as much as possible - so add option for bank account (IBAN in case of Europe)