From my experience, the problem is with SurePost or SmartPost, whatever UPS or FedEx calls it. When they hand off a package to USPS for delivery, the responsibility turns into finger pointing. And service drops like a rock!
USPS is definitely in trouble though - but we need USPS.
I don't know if the delays is their system or their people. In my case it's their people. I've had their postman not deliver packages, left a note for me to go to the next town and pick it up "because he didn't have room in his Jeep for it". That's not good service in my book - especially when I paid an extra $26 for home delivery.
As for Amazon vs eBay, there's no contest. Amazon has an impeccable guarantee. If you don't like an item for any reason (or no reason), you can return it for credit. Amazon backs the customer.
\With eBay - Ha! Lots of luck. And even if your purchase is received there's absolutely no assurance that it is not counterfeit, used, remanufactured, etc. And don't expect eBay to do much for you if you need or want to return the item. eBay backs the sellers. Returns will cost you dearly.
Nonetheless, one MUST shop out every purchase. Sometimes I find items priced better at the local brick & mortar store, and sometimes not. Time and cost are the key deciding factors.