Not shipppng to various zip codes

I occasionally have a customer contact me after ZenCart has told them that I am not shipping to their area. This is not true! I ship to anywhere in the world. I have looked all through the admin panel and can not find anything that would cause this. Someone must have run into this before. Can someone please tell me where I should be looking for a setting that change this?

Where are they usually from and what Shipping Module(s) do you have installed?

What kind of products do you have setup?

Have they said what kind of products they are trying to order when this happens?

Thanks for a quick response. I have been out of town. I have USPS shipping installed and have looked it over carefully but I do not see anything that should treat one zip code any different. This past month these people have been in 4 different states. The products are just electronic kits weighing only a few ounces. You can see the site's store at http://schematics4you.com/zc/.

What version of Zen Cart?

Clean install or an Upgrade?

How did you install? How did you upgrade?

Is your Zone for US shipping for All Zones or for specific states?

It was a clean install from the latest version. This was in November. I unzipped the file into a new folder named "ZC". I uploaded all the files via FTP to the server into a folder with the same name. The server is UNIX and my computer is MAC. I accessed the web based installer and went thru the process without a problem. Once installed I went thru each configuration page carefully and gave each option some thought before making any decisions. For shipping I installed UPS and USPS modules but later removed the UPS option through the admin panel. Payments are only thru Paypal. I turned on flat rate shipping at $9 Tax is based on shipping address and no zones. Probably you have even more questions now? I have attached a file from that page in the admin panel.