If the package was shipped by Fedex, then it was sold and shipped by a third party. Amazon stopped using Fedex as a carrier after their contract expired, and the latter declined to renew. Amazon even banned its 3rd party sellers from using Fedex Ground for a month due to performance issues, before reinstating that option at the beginning of the year.
Fedex Ground/Home is not the original overnight company of lore, Fedex Express. It is a subsidiary created when the company acquired RPS, a low-cost alternative to the other shippers. The SmartPost product, where the USPS does the final mile delivery, was also the result of an acquisition.
That's the simple story, but the takeaway is that there is a good Fedex, which you pay dearly for, and a not as good Fedex, which is cheaper.
Since it was shipped directly by the seller, Amazon was never in the chain of custody, and is limited in what it can do; not any more than eBay can do with its sales. Neither has a direct role in the shipping process, and has to rely on the tracking provided by the actual carrier.
The 48 hr grace period if there is a shipping discrepancy is a policy that applies to all Amazon sales, even for Prime members. They want to ensure that a shipment is truly lost, before taking action. Sometimes, the package turns up, and sometimes, it doesn't. It's sucks, but that's the way they approach it.
Like many customer service operations, Amazon would prefer to first funnel inquiries through FAQs, and automated systems that may help resolve issues, before having to get a human involved. As such, it stopped publishing its telephone number long ago, and the channels to initiate contact are a bit obscured. Buy, they do exist, in the form of online chat (preferred) or a callback option.
Delivery drivers are under enormous pressure, especially given the higher volumes during current times. Unintended mistakes can be made, and less scrupulous drivers can fudge with the process and enter false inputs, which result in false tracking results.
The USPS was created with higher service standards in mind, and its carriers don't have to learn new routes daily, and deliver to the same addresses every day. That's why the mailman is more likely to know where you live. But things can go wrong with them as well, so it's not necessarily the perfect benchmark.
Stuff happens. If it is a mission critical part, then it would have been wiser to pay the increased cost for the convenience and assurance, rather than see any savings lost to wasted time and aggravation. I've been there too, and had that lesson forced on me.