Purchase order- tax option per line item

Please make an option to add tax or not to a line item on a PO like you do on invoices. I don't know if other designers do this, but many times I have vendors tax me up front so it's easier for me during tax time. On top of that, I like to have my POs match my invoice from vendors, so when there is an installation line item that is not taxable, I'd like to mark that. Currently, I cannot add these line items without tax being added on. Thanks!