That's very different from my experience. I had 184 Amazon orders in 2023 (includes Amazon Fresh grocery delivery orders). Without spending the time to check each of those orders, I would estimate that around 90% of orders were delivered by Amazon. Only a small percentage were delivered by other carriers. My most recent delivery, a solar powered security camera so a small package, was delivered by USPS. It was delivered Jan 25 but USPS tracking's latest update still shows it arriving at the local PO the morning of Jan 25 (it's now the night of Jan 27). My point being that USPS tracking sucks as much as their delivery service. If USPS was to do what you mentioned, leave a notice to pick up at PO, I wouldn't do it, I'd request a full refund from Amazon and let Amazon deal with USPS. Another thing I dislike about USPS delivery is that I receive no notification and they don't ring the doorbell. With Amazon delivery, I get immediate email/text notification of delivery. UPS and FedEx ring the doorbell. USPS deliveries can sit around until I see it. I have motion detection on the porch but USPS doesn't step onto the porch to trigger it. Concerning your mention of USPS vehicle cargo capacity being a factor, I have had packages delivered by a USPS van, not by the letter carrier/vehicle, on occasion.
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Screenshot night of Jan 27, package was delivered evening of Jan 25.