Since USPS shipping method comes with Magento installation as one of basic shipping methods it is quite normal that we expect it to work ‘out of box’. That was correct until recently ‘WebShopApps’ didn’t patched Magento core module. When you open System->Configuration->Shipping Methods section and tab for USPS you will notice a small message that will inform you that module is patched.
First reported issue after patch is missing shipping options when you want to create a order form Magento back end, for now it is confirmed that issue happen with USA addresses. Quickest way to find out what is happening is to enable ‘Debug’ option and try to create one test order. Key step in this process is requesting shipping and payment methods, after you selected user and added products into cart. If USPS method is missing connect via FTP to server and search for debug file, it should in folder var/report by default. Open file with some editor that can handle XML output (I prefer Notepad++ – with nice plugin to format XML) and try to find what is causing issue.
So far most common error message is: RECTANGULAR is an invalid container type for a REGULAR package and ALL service.
To resolve issue go back to System->Configuration->Shipping Methods section and open USPS tab, under settings search for ‘Container’ type and switch it to ‘Variable’. Save changes and try to create order again, USPS should appear among other shipping options.
Issue is known for Magento from 220.127.116.11 up to 18.104.22.168, if anyone else notice similar issue but find different error and solution please shear ti via comment so we can help other people that are stuck.