Take the laptop back right away and have them make things right. If you wait too long, they may say you had another liquid spill. It can sometimes take a bit of time for the liquid to dry out and become sticky which is why the tech overlooked it originally. Plus if the sticky keys are used, then it may loosen the key up so it appears Ok, but after sitting a bit unused will cause the key to become "stuck" again.
Depending how the laptop was repaired, this can be tricky to resolve especially if the laptop was sent to an Apple mail-in depot (by Apple, or AASP)...usually the AASP must contact Apple to make a repair exception first (repair should be free if the original was a mail-in repair). If the shop performed the repair themselves on their own premises, then you should only be charged for the extra part needing replacement as I would hope labor would be waived for missing this part originally on the previous repair.