For what it's worth -

An "invoice generator" is an age-old and still pertinent inside joke in shareware circles.

Whenever someone new surfaces and says "I had enough, I want to write something of me own, I'm ready! Give me some ideas!", the answer to that is "do an invoice generator". It's a technically simple project that looks viable on the surface, but is ridiculously hard to market and monetize. It's a trap basically.