Defaullt PST exempt cutomer

Here's my problem after the newest update...

When a customer is PST exempt we have their customer record set to PST exempt and to charge only GST but about 10 times in a month this particular customer needs to be charged PST on an invoice, Sage will not let me manually change the tax code on the invoice from GST only to GST and PST.  When I called Sage Support this morning, their solution was to go into the customer record and change the tax code from PST exempt to GST and PST, issue the invoice then change the cusomter record back to PST exempt.  THIS IS RIDICULOUS. Does anyone have any solution for us?