On the first page of your Sell Your Item Form, you can make templates for your basic items. If these have no international shipping, you won't have to adjust every time.
When you go to International Shipping, use Choose Custom Destination and choose only USA.
Do not choose another Offer Additional Service.
Since you don't have a shipping fee for any other destinations than Canada or the USA, bidders will have to contact you for shipping costs. Most won't bother and the others you can refuse and Block (Manage your Bidders / Site Map).
