you'll need to buy either one more box than you actually purchased or you'll have two boxes too many.

Isn't that the truth? Last year I moved in the middle of Passover and had a box in the car with me. I ate about two pieces from it and it just sat in the car for ages after.

Will I beat the formula if I buy one box and then go back for one more? It fits the "one more than actually purchased" rule.