This error means there is a column named UserZip in more than 1 of the table being used in your query.
You query is returning column from the Orders, OrderDetails and Users tables. You need to specify in your link which table the userZip column to use is in. For example, to use the userZip column in the users table, your link should look like:
<a href="buynintendodsgames.php?sort=Users.UserZip">Zip</a>