Stamps That Make You Think

I can solve the first two Mensa stamps.

11 + 6 = 5 (subtract 12 from the sum)

143 (middle digit is the sum of the outer digits)

I'll have to think about the others. Thanks for the sleepless night, Germania!

EDIT: Cambridge 90 (number of letters in the word multiplied by 10)

EDIT: circle = 8, star = 10, triangle = 3, so circle + triangle + circle + star = 29.
This is not really a test of intelligence, but of how well one remembers the technique of solving simultaneous equations.

Perhaps the most well known examples of ignoring the UPU are 331 and 332 for US stamps. Values had to be numerals not words. That's why only those two were issued; there are essays for other values.