Maybe I am overthinking this.
- Invoices do not contain addresses ever.
- Internal (intercompany) can have a blanket code to exempt those Customers from all reporting and taxes.
- International can be handled with the countries themselves (I had completely forgotten about that)
Therefore, get the country, customer, Ship To and order releases (OTS) right and that should cover it. Meaning:
- Enforce Country to be non blank on orders and customer setup
- Enforce Countries that are not US and Canada to use your international Liability code
- Explain to people that Puerto Rico is part of the USA
- Clean up any existing data
- Do not allow editing of Tax Liability by the users ever. Not on invoices, not on orders, not on customers. Never.
Only issue could be a CM of an invoice that was messed up before…