Did you ever implement this?

We are thinking about it, but in order to add a fee that is a % of the total, the sales order total would need to be known first.

The second place we would need to add it is to credit card payments (of existing invoices), which would be a manual calculation.

I did scripts for this 4 years ago to work with sales order data entry, invoice data entry, shipping data entry, and cash receipts entry. No changes have been required since. I added a UDT for necessary state criteria. Local jurisdictions weren't required in my situations. To determine the total, I did these in the Pre-Totals event so that the inserted charge could be recalculated if the order is subsequently changed.

I did scripts for this 4 years ago to work with sales order data entry, invoice data entry, shipping data entry, and cash receipts entry. No changes have been required since. I added a UDT for necessary state criteria. Local jurisdictions weren't required in my situations. To determine the total, I did these in the Pre-Totals event so that the inserted charge could be recalculated if the order is subsequently changed.