What these have in common
They do arithmetic and nothing else. None of them knows your tax rate, your payment terms or what you're allowed to charge. You supply those, because they depend on where you invoice and on what you agreed before the work started.
None of them stores anything. Nothing is written to your browser's storage while you use them and nothing is sent to us, so the figures are gone when you close the tab. The one exception happens only when you ask for it: Create this invoice hands the figures to the invoice generator in the same tab, which saves them as a new invoice alongside any you already have.
Not arithmetic, but before you pay one
An invoice asking you to pay into new bank details is the usual shape of invoice fraud. The invoice scam checker goes through the warning signs and compares the sender's address with your supplier's real one. It and the other tools that write letters or check numbers, rather than do sums, are on the tools page.