USPS has recently changed their rates effectively from 7 september 2014 and included some welcome postage savings when you ship packages. Now a days most of the zen cart stores are not working properly with USPS, and in shipping details USPS rates are not appearing.
This issue is occurring on all the websites which are based on old Zen Cart versions v1.3.x or an old zen cart USPS module i.e. RateV3.
Thanks to zen cart team, this issue can be resolve by updating the USPS module from RateV3 to RateV4.
This module can be download from official zen cart site here http://www.zen-cart.com/downloads.php?do=file&id=1292
This module supports Zen Cart v1.3.9, v1.5.0, v1.5.1 and use latest rules of USPS i.e. Domestic RateV4 and IntlRateV2.
This module returns real-time rate quotes for customers to choose a better USPS service on shipping options screen at front end checkout.