We understand you have purchased a fare inclusive of a complimentary snack. If you would still like to receive a snack, please make yourself known to cabin crew once on board and they will be able to assist.
If you paid for the snack in the price of the ticket then it's not complimentary.
If you paid for it you shouldn't have to ask for it. If they can identify you enough to send you the letter, then they can identify you on the aircraft and provide you what you've paid for.