USPS has weight limits, their carriers are trained to deliver anything within those weight limits.
If they can't deliver an item that meets their own USPS requirements then that's their own fault not yours.
If they can't deliver an item that meets their own USPS requirements then that's their own fault not yours.
I know multiple people in different cities who have been told that their particular USPS mail carrier couldn't carry some reasonably-sized packages, so they wouldnt deliver them to the home.
This is not a case of "these packages are above the limits we allow for home delivery" - it was very clearly a matter of the carrier for that particular route not wanting or being able to carry those packages.