I believe that customers are members of pricing categories and that you have a different set of records in products for each pricing category.
If that is correct, then there are two changes that you need to make:
Remove Pricing Category from the relationship between Customers and Invoices. It should only match by the customer id field.
Go to field options for Invoices::Pricing Category and set it up with the Looked up value auto-enter setting to copy Pricing Category from the customers table.
Thanks. I'll give it a try.
I tried what you sduggested but it did not work. Any other ideas?
Try it with a brand new invoice record. It will work if correctly set up.