The USPS is going to suck for as long as anyone here can see and it won't expand 7 day delivery much beyond what it delivers now, if at all.
The USPS is still looking to cut full service back to 5 days a week in order to better align paid hours with available work. Amazon accounts for a tiny percentage of their volume and the Sunday freight is overwhelmingly delivered by lower wage temp and PT employees.
And then there's the clause in the contract that allows the USPS to exit the arrangement for any reason with a 30 day notice.
Well they seem to be unsucking more, perhaps shorter run from regional warehousing is the ticket. As mentioned, we'll see about the rest.