Free tool
A tax invoice with everything on it.
Most invoice problems are not design problems. They are missing-field problems, and they are why an invoice gets queried instead of paid.
What a tax invoice needs
The words "Tax invoice"
Sounds trivial. It is a requirement, and its absence is a common reason an invoice gets bounced back by a client's bookkeeper.
Your identifiers and theirs
Your business name and tax number, their name, and a unique invoice number that does not repeat.
What was actually done
Itemised, with dates. "Services rendered" invites a question and delays payment.
Tax shown clearly
The tax amount, or a clear statement that the total includes it.
Terms and how to pay
A due date and the payment details, on the invoice itself rather than in the covering email.
Requirements vary by country and by the size of the invoice. Check the rules for your jurisdiction — this template covers the common Australian case.
Questions people ask
When does a template stop being enough?
Usually around the point where you are editing last month's invoice and cannot remember whether you sent it. A template does not track what has been paid, and that is where the money actually goes missing.
Do I need accounting software instead?
Not necessarily. You need something that raises the invoice, sends it, and tells you when it is late. That can be accounting software, or it can be something that does it for you.
Or stop writing them entirely.
Describe the job in a message and the invoice comes back ready to send.