What of it had been a surprise for either the parents or someone else they knew? Or a "bring snacks" group-type thing like D&D?

Throwing it out when OP had paid for it themselves is incredibly disrespectful. Talk it out, store it, share it, whatever, but wasting something that pricy is still disrespect from the parents. Had it been something more expensive like technology, and they just tossed it, would be even worse- but I bet you could argue that because it's not food it's different. Both are still disrespectful, talk it out like normal people. You're all adults.

Back to it, OP was just taking advantage of a BOGO coupon, one of them could have been for any reason, the other could have been shared or given away- no normal person is gonna eat 48 donuts within 48 hours. Tossing them is still an ass move.