If you want an app that automatically and instantly invoices your customers with all local sales taxes/VAT etc. calculated and shown, Sufio Automatic Invoices does what it says on the tin. It has all the bells and whistles and customization facilities I need. My store sells digital products and Sufio links perfectly with it and with the digital products app I use, and picks up on any discounts issued as well. First class app, very well thought through.