Always remember there is Regional Rate A & B boxes ( you have to order on line if you have a USPS account! ) If you are sending to 1 to 4 zones from you, prices are cheaper then "If it fits it ships Priority" flat rate! Most folks do not know about this!
they also have just priority boxes that look exactly the same as the flat rate boxes but are a titch bigger, they are like 28$ for 10 lbs.
so you have to watch which ones you grab.
I believe they use regional rate stuff where they can to save shipping also . I lived "only" 600 road miles away but less than 150 road miles PO hub to hub so Sunday night in his PO could have it in hand Tuesday . 3 day service was typical .
I remember when a 5-10# box was $18-22 and UPS was the only reasonable insured and tracked way to ship stuff . Weight has always been the killer . I shipped a box for my folks that just barely slipped in diminsionally at 115# for UPS . It was $96 . The stuff packed of course wouldn't have fit in any of the FRBs but the 115# could have shipped for about $26 then . Still can't kick about about 60# with 3 day service for less than $20 . It cost almost that for a 1oz letter from St Louis to Sacramento to get that kind of service on Pony Express .
I have always found that strange I can't believe he gets away with that That envelope is for flat documents only. Around here my post offices would not allow that.
Also keep in mind that you can first class parcel any type package if it weighs 13 oz or less!
13 oz parcel is about $3.70 ( lighter weights are less money) The interesting thing about this is package size doesn't matter.
For instance some 2 cavity lee moulds ( without original box) can be shipped in a generic "tyveck" envelope and be 13 oz to ship first class parcel That is only $3.70 I ship small amounts of range brass of odd calibers this way!