So ISO A1 then. I plugged in something like 24x33x5 as well as the Amazon A1 size (which is smaller) and got the same price range but with additional options for the smaller (Amazon A1) sized package. At that size, the weight is the determining factor at USPS (at least based on the rates I've seen) even though 24x33 is still considered an oversized package.

OTOH if you want to ship it express (Global Express Guaranteed — GXG in USPS speak) that comes out to around $265 (less if you pay online) and not all post offices will take GXG packages. $60 to $265 is the difference between a 1-3 business day expected delivery time and 6-10 business days.