I found these solutions:
-If there isn't a space in the code and there should be then add it, and vice versa.
-Make sure you have previously selected the country that the book is to be delivered to before you insert the post code.
-Make sure there isn't a blank at the front. delete and retype,
-Also, if you're using MS Internet Explorer try a different browser.
Also you could try to change the shop from the lower left corner to some other than US.
-If there isn't a space in the code and there should be then add it, and vice versa.
-Make sure you have previously selected the country that the book is to be delivered to before you insert the post code.
-Make sure there isn't a blank at the front. delete and retype,
-Also, if you're using MS Internet Explorer try a different browser.
Also you could try to change the shop from the lower left corner to some other than US.