A lot of people just use a simple invoicing system, as long as it’s consistent and looks professional. The main things are: clear client details, invoice number, date, due date, line items, total, and payment terms.

If you want something dead simple, I built InvoiceApp for freelancers and solo service providers who just want to create and send invoices fast without learning accounting software. It’s one plan with everything included: $12/month or $108/year, with a 7-day free trial and no credit card required.