A free online invoice generator is the right tool for an invoice that has to go out today. It is the wrong tool for the tenth one. A web page has no memory. It cannot tell you which number you used last, which invoices are still unpaid, or what you billed this client in March, because nothing typed into this page is kept anywhere.
Once invoicing stops being occasional, the work stops being the writing and becomes the keeping track. What was invoiced against which job. What you quoted first, and whether the two still agree. What deposit was taken. Who has paid, and who is three weeks late. A folder of PDFs beside a spreadsheet is where most people find out that does not hold.
That is the part Zeus does on your phone: every invoice raised against the job it belongs to, the numbering kept straight, deposits and part payments tracked, and the outstanding total sitting there without you adding anything up. It works with no signal, so the invoice goes out from the driveway instead of from the kitchen table at ten at night. The free plan costs nothing and asks for no card.