way back when MPC was under "Fundimension", a part of General Mills, they had a "savings stamp" program that let you cut proof-of-purchase logos out of the kit box and send them in for free kits....
and after MPC was absorbed into AMT, the program covered both lines, although i THINK only the MPC branded kits had the tokens on the box.
anyways; i had dozens of kit boxes stored away (nothing really collectible, thankfully) and i went through and clipped every one of those tokens out, shopped the little "catalog" for kits, and sent off a scad of tokens one summer in the early '90's....
AMT sent me six kits total for my efforts. i think i ordered their venerable '49 Ford, as it was not currently on the store shelves, and probably a couple of the excellent new '68 Road Runner kits.
would it kill one of the kit makers to do something similar today? say, scan hobby shop reciepts showing you paid full price for kits and earn some credit against a "mail-in" kit?
pie in the sky, i know, but it was a good memory.
BTW, the only per se "collectible" kit box i have (in my opinion, anyway) is my dusty old JoHan Petty Plymouth box. there's a Boot Hill Express stored in it.