Yep, looked up values are the way to go here.
Define a new table, SalesTax, with two fields: State, TaxRate
Define a relationship linking this new table to the State Field in your shipping address:
Invoice::ShippingState = SalesTax::State
In your Invoice table, define a matching number field, TaxRate, bring up the field options dialog, select the Auto-enter tab and select the looked up value option.
In the dialog that pops up select SalesTax::TaxRate as the field from which a value will be looked up.
Now, when you enter or select a state in Invoice::ShippingState, Invoice::TaxRate will copy the current tax rate over from the SalesTax table.