As any fashion buff will tell stylishly tell you, the word costumes refers to a type of dress specific to an occasion, season, nation, region, or historical period.
The word customs refers to the usual ways of acting in a given circumstance, like a holiday.
Similarly, costumers are people who outfit other people in costumes, like on a movie set. Customers are people who buy things, like at your job at the commemorative spoon store.
If you switch any of these up, the results can be hilarious. But since that's probably not what you're going for in your writing, it's a good idea to keep costumes and customs on your spelling radar.