Hi Saints-rlfc,
Based on your description of the error you are receiving, I suspect the API warning is referring to what you entered in the "Zip Code" field in the "Shipping Tools" section of the listing creation page. However, we would need to take a look at the listing in question to form a definitive conclusion.
If you are still in need of assistance with this issue, please contact our customer support team using the web form on the following page:
http://www.auctiva.com/help/requesthelp.aspx-Mike