GST code on sales order should default from Customer Card not Item Card

We have items which have 'GST' set up on the Item card.  Then we sell these items to both Australian customers  (GST code on the Customer card), New Zealand customers (NZD code on the Customer card) and other overseas customers where no GST applies (EXP code on the Customer card).  However whenever we raise a sales order, the GST code defaults to the code set up on the item which is GST, and we need to remember to change it for New Zealand and other overseas customers.  The GST code should in reality default to what is on the Customer Card - can this be changed please?

The same applies on the purchasing side, sometimes we buy the product locally and sometimes we import it in.  Therefore any purchase order we raise should default to the GST on the Supplier card, not what is on the Item card.