> I wonder if it is because USPS Weekend delivery isn't the normal "route" person, and they just don't try to get into multi-unit mailrooms?

I'm pretty sure they don't try to deliver anything at all. Every time I've had a Sunday order and everyone else that I know, it gets marked as either "Delivery Attempted" or sometimes "Delivered" without the package arriving. I've NEVER heard of anyone actually receiving a Sunday USPS delivery for an Amazon order on Sunday. Literally never!

I think the Amazon/USPS deal has the latter paying a penalty for late/undelivered packages. It's possible that they don't have the resources to actually deliver the packages and lying (either from corporate or from the actual mail carriers) is the only way to cover their asses.