Luckamancy charms: They're Tragically Malicious!
 Proposed Canon
The box contained a prize, the third and final piece of Parson's Sword -- the foible (the pointy end of the blade that hurts things).
On the front of the box was shown Hamstard, a white blobby or ghostly form, and an example of a bowl of the cereal with the Mathamancy Gauntlet, the 3d glasses and the sword (completed) in the cereal like marshmallow bits.
The back of the box (pictured) consisted of a maze transposed on a volcano similar to Gobwin Knob with a green dwagon flying nearby.
The box claimed that it "contains a full day's supply of Luckamancy", and Parson later acted on the assumption that it conferred to him Luckamancy, trying to cast a Luckamancy spell on Bogroll before sending him on a suicidal mission. There was no evidence of successful casting.
 Real World References
This Stupid Meal is based on Lucky Charms, a cereal consisting of oat-based rings and marshmallow bits in various colours and shapes with a "luck" theme -- green clovers, purple horseshoes, yellow stars, etc. The mascot is Lucky, a leprechaun: it is this character that Hamstard mocks in his green hat.