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:
 
 - Go to Lists menu at the top.
 
 - Select Templates.
 
 - In the Templates window, select the invoice template and click the Open Form button.
 
 - In the Create Invoices window, go to the Formatting tab.
 
 - Click Customize Data Layout.
 
 - In the Additional Customization window, go to the Columns tab.
 
 - In the Other 1 section, put a check-mark for Screen and Print box.
 
 - In the Title field, enter Rep.
 
 - Click OK.
 
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!