I believe, and please correct me if I'm wrong, that in a pyramid scheme the individual participants receive money from new participants that they recruit (and the people your recruits recruit, and so forth down the line). If you join a pyramid scheme and then do nothing else, you receive nothing.

A Ponzi scheme is centralized, with new participants joining directly with the scheme, and proceeds paid out to all existing participants.

As structured, this sticker scheme is a Ponzi scheme, since it's all centralized and you don't do anything besides send in your sticker and postage.