Choose Free Shipping as your shipping price for domestic items.
You may also make the shipping fee to the USA $0.00, but don't offer Free Shipping overseas. (It's just too expensive.)
And you know that Free Shipping really means that the cost of shipping is included in the price, eh?
Which is cheapest:
A $5 item with $10 shipping
A $10 item with $5 shipping
A $15 item with Free Shipping?