What happened is USPS wanted a piece of Amazon’s business so Amazon said “OK, but you have to take everything we give you”. USPS is capable of delivering parcels but having worked on projects in both I can tell you the are horse-and-buggy vs. a Maseratti compared with the parcel delivery companies. The facilites are highly automated but it’s all geared to handle letter and tray mail (500 letters in a tray) or flats (magazines, catalogs, etc). “Smalls” are automated but large parcels are handled manually.
Which is why it makes no sense for USPS to be required to handle large, heavy, bulky packages - they are not set up to handle them from end to end as FedEx and UPS are, and cannot charge what it would take to cover their costs when they are required to meet the package delivery company prices.
I couldn’t find what I needed locally so I ordered a chair mat from Amazon - about 4’ square plastic mat that you roll your office chair around on. My poor USPS rural deliver person had to tote that thing around in her Jeep and haul it up to my front porch...
Thank you for that information.