i was going to suggest being able to search for an invoice by keyword but thats being suggested already so i have ticked that one...
also what would be good is when searching through invoices it would be good if there was a next invoice arrow or box so instead of clicking on the back button to go back to the main page then sort by date, and remmember what invoice you were up to, if there was a next invoice it would open the next invoice when your alrready in a invoice (or a go back one invoice or go to next invoice).
"A next Invoice button."