Shipping rates

One flat rate per order, and free shipping over a threshold.

You set one rate for the US and one for everywhere else. It is charged once per order however many items are in the cart, because charging a second time because someone added a second shirt is the fastest way to lose the sale.

Free shipping

Set a free-shipping-over amount in Settings and shipping drops to nothing once the basket reaches it. Leave it empty to always charge. A threshold a little above your average order is the usual way to lift basket size.