Post Office is one of those party
kissing games. It usually isn't played anymore, but occasional references to it are made by older generations.
There are two groups formed. One group is all females. The other group is all males. One group will go into another room and organize themselves as if they are Post Office employees. Then, members of the other group come into the Post Office and receive kisses from each of the employees.
So basically... every person kisses every other person of the opposite sex.