We have a headquarters address that is used typically as the ship from when invoices are generated. In mixed sourcing states this is making tax calculate incorrectly as the headquarters is not the real ship from address.
The Avatax engine being a purely data in = data out type of service, the only way we can calculate tax with a specific ship from address is if that is what Avatax is sent by the accounting software for the invoice itself.
It is possible in various accounting software to use a warehouse address for example and associate that as the ship from instead of the typical headquarters on an invoice by invoice basis.
This depends completely on the abilities of the accounting software you are using, and is best discussed with your accounting software administrator or IT department.