On September 28, 2014 the United States Postal Service will end support of their Rate V3 API.  This was the interface used by ShopSite and other shopping carts to get real-time shipping rates from USPS.  Fortunately the V3 API has been replaced with V4 and your store will continue to work if you are using this interface.

ShopSite v12 (in Manager and Pro stores) supports either the Rate V3 API or the Rate V4 API.  To confirm that you are using the Rate V4 API go to Commerce, then to Shipping, and click on the Configure button for USPS.  Under the Rate API Version make sure that V4 is selected as displayed below:

That’s it.  You are good to go with ShopSite v12.

If you do not have this “Rate API Version” selection then you are running an earlier version of ShopSite Pro or Manager.  Versions prior to ShopSite v12 only supported the Rate V3 API.  If you are running ShopSite v11 there are patches that will switch ShopSite to use the Rate V4 API.  To determine the ShopSite version you are running look at the bottom of the page on the main (dashboard) screen in the back office (merchant interface.)  You should see something similar to this:

On the left of the page you can see that this store is running ShopSite Pro version 12 release 2 (12 r2).  Also on the right you can see the large ShopSite 12 logo indicating that this store is running version 12.   If you cannot confirm that you are running ShopSite Pro or Manager on Version 12 and you are using USPS for real-time quotes, contact your host about upgrading you to ShopSite v12 or applying the v11 patches.  You can refer your host to this knowledgebase article:

May 8, 2014 email from USPS regarding USPS Web Tools Rate V3 retirement on September 28, 2014