What is the ruling on drinking a beverage obtained in a way that violates the store's conditions, and what is the ruling on drinking the amount that the questioner himself brought?

The condition in purchasing is probabilistic. If the intention is that a person should not buy more than two boxes at a time, with the possibility of returning or sending someone else, then there is no harm. However, if the condition stipulates that the customer benefits from the discount only once, without resorting to deception by returning or bringing family members, then this condition must be adhered to, due to the generality of the Hadith (definition): "Muslims are bound by their conditions in what is lawful." In this case, it is necessary to seek absolution from the shop owner by paying the price difference if he does not absolve you both.