I like it!

Here's a few suggestions you could think about.

- Link the generation of invoices to existing apps/online services. No invoices work alone, probably you need to tap onto existing apps/services to help simplify users's adoption of your app.

- Er.. why are you not making $$$ from it? Have a paid and a free options. One way I can think of the paid option is to allow users to save generated invoices.