Hi there, @Mark.

You can have a custom field permanently on the invoice sent to customers. Like my colleague @Michelle_b stated earlier, must you go to the custom field tab in the picture, and have to make sure to have the master invoice or the invoice used to send to the customers. I've got you a link to a video that will help understand these Custom Fields and how they work.

Please know if you have any other question, I'm always here lending a helping hand. Happy Hump Day!