How to bypass VAT calculations by the Billing Interface process

We are currently populating the billing interface from sources, like OM, Billing Excel Upload and also some third party systems. Some of these third party systems are using a external tax engine to calculate taxes for invoices produced. The VAT setup in these systems is not necessarily the same as in PeopleSoft from a VAT coding perspective etc. I would like to know how people have managed integrating invoices with VAT without PeopleSoft recalculating it. We currently have a workaround in place to book the VAT on an invoice line (so setting the invoice to out of scope) and directly