Telling the cashier they forgot to ring up the drink saves their shift, not just your conscience.

Flagging an error that favours you gets framed as being good, and the practical effect lands on one specific person’s shift. Tills get counted, shortfalls get noticed, and the cost attaches to whoever was standing there. Mentioning it is less a moral gesture than a refusal to let someone else absorb your luck. It also takes about four seconds.

Say it before you leave, plainly and without drawing a crowd. If you only realise later, go back when that is realistic. Either way, the reflex is worth building.