Are you using the Advanced Sell Your Item form?
It's actually a little easier than the Simplified form eBay defaults new sellers to.
You may also want to do this on a computer rather than a phone. If you don't have one, try the Public Library.
Now.
Go to -> Shipping Details
Go to-> International Shipping
Go to->Ship to
Now- you have enabled Custom Shipping.
And you have enabled USA
Right?
You will also see another Custom Shipping menu.
Open the menu and choose "-" instead of Custom Shipping.
(The Custom Shipping should be showing UK chosen now and you are ending that).
Double check that you are not paying for International Visibility. You don't need it.
Scroll down to Excluded Shipping Destinations.
Click on all the buttons EXCEPT North America (which would exclude the USA).
Best wishes for your future business.
