I got a beautiful mailed piece form Sun Country the other day...I signed up for their "ufly" rewards program and the intro packet was really nicely done. It's navy blue and orange, square in shape, and unfolds into what amounts to a flower shape, with a glossy plastic membership card inside and a blurb about their credit card and a rebate to cover TSA pre-check fees. They seem a little low-tech, there is no mobile app for the ufly program.
Aside from mailed pieces I've received from AMEX pertaining to my Delta Skymiles and Hilton Honors cards, I don't recall getting mail from any airlines I've signed up with for rewards...emails by the dozens, but no mail.