Send it when the work is done, not someday
Include your legal name, the brand, a description of the deliverable, the amount, the date, and payment terms (for example, due in 15 days). Attach the contract or the email that agreed the price. Use a method you can track.
If they go quiet, reply once with the invoice attached again and a date you will follow up. Keep the thread. This is also why a separate account helps. You can see who has not paid without scrolling a personal debit card.