For the purchase you need to use a dummy item or tax account directly
For invoice go to Setup nexus
On the us nexus , check available Per-Line Taxes on Transactions – Check this box if you want to assign taxes to transaction line items. This enables you to charge different tax rates for items on the same transaction. Each time you add a line item to a sales transaction, you must select the appropriate tax code or tax group in the Tax column. You can add taxes to line items on the Billable Items, Billable Expenses, and Billable Time subtabs.