You do know that North America includes Mexico, right? And Bermuda?
The way to add USA to your shipping destinations is on the Sell Your Item form.
Under the domestic shipping is International Shipping.
There is a dropdown menu. Open Custom Destinations.
Choose USA.
Enter the service (flat or calculated and the specific service, Light Packet, Expresspost , etc.)
If you choose Flat Rate, enter the price.
If you choose Calculated scroll down and enter the dimensions and weight of the package.
DON'T choose N&S America. This will override the USA pricing.
And of course, list in US dollars. The world understands the US dolllar and Americans understand nothing elsel.
Items priced in US dollars will appear on the dotCOM (US) site.