E-commerce website is not showing shipping rates

Our e-commerce website is not providing correct UPS, Fedex or USPS shipping rates after entering a US zip code. This is an e-commerce website based on Drupal and Ubercart, with considerable customization. When customer enters a US zip code, the website is supposed to send an API request for shipping rates to UPS, Fedex and USPS. The request includes de destination and origin zip codes, package weight, package dimensions, etc. Since January of this year, the website stopped showing shipping rates. Instead it sometimes gives the error: "please re-enter zip code", or the page hangs up, or it shows some rates which are irrelevant (ouside the US), or it shows shipping price $0.00. Either the website is not sending the API request, or the request is not being sent to the right url, or the request doesn't have the right format. (The shipping companies sometimes change the requirements for the API rates request). To fix the problem, we need to check if the website is sending the API requests, research the correct format, contents and url for the request, and modify the request as needed. The problem would be solved successfully when the website works as it did 2 months ago: upon choosing an item to purchase and entering a zip code on the website, the website should display within a few seconds all the relevant UPS, Fedex and USPS rates available for the zip code entered, as well as the arrival dates. The rates should be ordered from cheapest at the top to more expensive at the bottom. Also, if we block one of the shipping companies (UPS, USPS or Fedex) from Admin, the website should display the rates for the other companies that are still active. We need a freelancer who is able to troubleshoot php code directly. This is not a problem that can be solved from the website's Admin module, because of the customization of this website.