Hello there, @lousch.

I appreciate you for providing a detailed response to your concern. I'm here to share some information regarding adding payment terms in QuickBooks Desktop.

Based on your screenshot, you're trying to add a payment term to a Job. Currently, you can only add this type of information directly to parent customers.

Check out these screenshots for your visual guide:

As a workaround, you can add the terms manually when creating sales transactions. There will be a drop-down option for this information (see additional screenshot below). 

I've also attached an article you can use to assign expenses to jobs in QuickBooks: Track job costs in QuickBooks Desktop.

Fill me in if you have additional questions or concerns about managing customers and jobs in QuickBooks. I'm always here to help. Take care always.