Thanks for getting back to us, sparsell.

When adding custom fields into the invoice template, you'll still need to enable them from the Additional Customization screen to be visible. However, the information that you need to enter in these fields is manual and no drop-down options.

Here's how:

1. Click the Lists tab at the top menu bar, and select Templates.
2. Double-click the template.
3. Choose Additional Customization.
4. Go to the Header or Columns tab.
5. Search for the custom fields.
6. Put a check mark on the box below the Screen column.
7. Click OK.

I'm adding some sample screenshot below for your visual guide:

In case you encounter issues customizing a template, I recommend visiting the article below:

Troubleshoot common issues when using and customizing templates.

Please don't hesitate to reach out to me if you have additional questions or concerns about the templates. I'll be around to help.