One of the final nails in the coffin for me was when I tried to specify the delivery date to a day I would actually be in, and they completely ignored the instruction and delivered immediately. Like, I pay for that option, and you're not delivering the service.
I've also been getting free trials for Prime every so often, so I still get the benefits when I might need them (eg before Christmas). They did make it very hard to cancel, though, half the links didn't work and the instructions were out of date and didn't match the pages.
No idea whether or not it was a branded truck (it was a while ago), but it wasn't under any other sort of brand - Amazon deliveries are done by people employed by or directly contracted for Amazon.