I remevber about 24 year ago, working part itmne at the post office plant in Tornto, and we would ship literally tons of mail by air. Mostly bills. In fact, we sent a record of 11,000 kilos on air just in one shift in our western despatch area, to Vancouver, Victoria, Edmonton and Calgary, in the busy week after Remembrance Day.
In my mind, I don't see these as needing that kind of priority, at leat not today.
I would guess the fuel used for air mail of first class letters in the USA must be staggering. Why not just use Xpresspost (in Canada) or whatever equivalent is used inthe U.S. for items needing air service?

