Fix shipping issue in Drupal-Ubercart e-commerce website

This is an e-commerce website based on Drupal and Ubercart, with considerable customization. When customer enters a US zip code, cart shows message: "Please re-enter zip code". Re-entering zip code doesn't fix anything. Normally, this website should get shipping rate quotes from UPS, Fedex and USPS in real time as soon as customer enters a US zip code. I am looking for a freelancer who is able to troubleshoot php code directly. Because of the customization of this website, the problem cannot be solved from Admin.