Custom Ecommerce Site - Shipping Rate API Integration is Broken

I run a small ecommerce business in the US. Our site was built a few years ago by a freelance developer (Laravel backend, custom frontend) who isn't available anymore, no docs left behind either. The main issue right now is our shipping rate integration. We pull live rates from a carrier API at checkout but it fails intermittently, sometimes it times out and just shows a flat fallback rate, sometimes it charges customers the wrong amount because it grabbed a stale cached rate. We've had a handful of complaints and at least one order where we basically ate the shipping cost difference. I don't want a rebuild of the whole site. I need someone who can actually go in, understand how the current integration works (or doesn't), and fix it properly, meaning real error handling, sensible caching so we're not hammering the carrier API on every page load, and a fallback that makes sense instead of silently showing wrong numbers. What I'm looking for: Review the existing codebase and figure out why the integration is unreliable Rebuild the shipping rate calculation logic so it's actually dependable, proper error handling, retries where it makes sense, caching that doesn't go stale Make sure failures are logged somewhere so we know when something's wrong instead of finding out from an angry customer Test it with real order scenarios, different weights, zones, package types Quick writeup of what was wrong and how the new version works so whoever touches this next doesn't have to start from zero Please share an example of an API integration you've built or fixed before, ideally something with retries, caching, or error handling involved. Not looking for someone who's only worked with drag and drop site builders, this is a custom codebase. If this goes well there's more work down the line, we have a few other things on the site that need attention