> how are they showing up in USPS and UPS databases?

Are you overestimating their validation capabilities? Maybe the validation just isn't that accurate or isn't always done, for efficiency.

I once desperately needed a (FedEx or UPS) package that was delayed. Tracking showed it in a facility about an hour away so I asked if I could pick it up there. I was told: 'It's not really there; our (customer-facing) tracking shows where packages should be.'

Their image of super-efficiency is for selling themselves. (If you are deep in e-commerce, you probably already know.)

Yeah USPS definitely reuses tracking numbers, but how were these guys able to overwrite the delivery address? I feel like this shouldn’t be possible unless they are inside the system.