Yeah I saw that on the first run, that you could charm the merchant with no downside. Afterwards it was like for any elf or half elf, why would you choose any other cantrip when that one is clearly more useful.
Part of me really dislikes the idea of charmed to hostile without being attacked first. Or like if the PCs bounce, I always kind of pictured it like as long as you walk out the room and the character is going to remain charmed for a while, maybe they don't fully realize what happened (depending on their wisdom or int) and so perhaps the Party gets away with it. But you'd think next time they came back to sell something, that character would probably have a much more guarded reaction. I think most merchants should have protections against being charmed, and be sort of magically paranoid about it. That way if the PC is about to try and cast a spell that might totally hose their ability to buy cool stuff later, maybe they get checked with a warning.
