Actually, UPS does: https://www.ups.com/us/en/services/e-commerce/efulfillment.p...
"UPS eFulfillment combines the fast and reliable shipping consumers demand with the scalable inventory management your operation needs. We can store the products you sell and get them into your customers’ hands quickly. Our eFulfillment platform and bundled pricing helps you manage inventory, orders and costs so you can focus on growing your business."
N.B. It's a 14 year old article at this point. Things may have changed.
 Or if you're a New Yorker subscriber: https://www.newyorker.com/magazine/2005/04/18/out-in-the-sor...
I'm going to guess UPS keeps warehousing separate per vendor.
You have a product with a UPC. You have a picker with a UPC and a scanner. If the scanner scans the wrong UPC, the picker grabbed the wrong thing.
And yet I get sent the wrong item from Amazon multiple times per year. Ballpark of 1 in 20 orders. And not like I got the wrong order. More like I ordered spicy taco seasoning and receive regular taco seasoning. So a mistake that makes sense if you were matching on titles but why not match on UPCs?
Amazon will only then use your items for your sales and not for anyone elses. Possible longer shipping times, etc. But I s always been possible to ensure correct items on the seller side with FBA.
Most people like to use the same SKU to get more exposure on amazon.com though.
If FedEx/UPS were to simply view inventory as packages awaiting an updated shipping address, and outsource a catalog service, they could easily compete.
Post office staff here are surly. UPS and Fedex drivers are bright and friendly.
Obviously it varies by location.
The spam is a profit center for the USPS, it's what allows them to deliver other mail cheaper than UPS/FedEx.
UPS, FedEx and DHL will leave packages in the open, if they deliver them at all. I work from home, and often they would claim delivery attempt when no such attempt was made. If they do happen to attempt and a signature is required but I am out, the nearest pickup location is an hour away.
Whereas Canada Post always attempts, and if I am out they will leave the package at the nearest post office, which is a short walk away.
Maybe USPS would have more value if it weren't being run into the ground? Canada Post performs reasonably well, after all.
Have you interacted with UPS's technical support on anything? They're a disaster, and aside from the example I mentioned have in the past waved off other bugs as being expected.
Have you tried shipping in general using UPS's website? You need to understand a lot about their system to really use it (for a simple example, the difference between "UPS account" and "having a working login under which you've provided your EIN").
UPS is okay but they don't have the key so often can't get into the building.
Fedex just straight up lies about deliveries and often never attempts them. By far the worst carrier.