At what distance would it ship by air (if it does)? Any ideas on expected shipping time when the distance is 1500 miles?
Priority Mail is not Express Mail.
Unless they changed the rules, Priority Mail is tracked, and moves with First Class Mail, but gets handled by each facility first along the way. Since by truck, coast to coast is about 48 hours on the road, it's normally three days, but they could air it with other Express mail.
Computers handle a lot of the routing, but most Priority Mail is ground shipped.
The delivery date is not guaranteed for Priority Mail.
But, if a large mail set, such as this time of year, slows First Class, the fast movement of Priority at each facility helps.
That was years ago when they defined Priority Mail and could have changed it to go air more often.
If it's a pizza, it will be there in 29 minutes or less.