Hello there, ronfisher.
I'd be happy to share the steps with you on how to add the Rep field on your invoice template.
Here's what you'll need to do:
From there, upon creating an invoice, you'd now able to enter the Rep in every line item.
For additional reference, I'm attaching some articles that you may find helpful:
I'd appreciate if you'd able to update me on how things goes. I'm always here to offer additional assistance. Have a great day!