Stupidly simple invoicing should be easier than this. probably invoice dragon is closest to that. What you should focus on as an additional layer to stupid is database and recordkeeping. "save an invoice" -> 'list of saved invoices" -> "database of invoice, order, product, customer, [etc]".

And use logins for storing this data online, with customer info, and product info. Offer fancy templates under paywall. Offer (monthly) reports under paywall. Offer data retention under paywall.

Almost there :)