Costumer and Customer are easily confused words. I’ve seen them used incorrectly in comment sections too many times to count.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know what word you wanted. It can’t guess what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass.
Autocorrect suggests words that start with the same letters. It suggests what word you may want to save time, but quite often, its suggestions couldn’t be more off base. Often, Autocorrect produces humorous results, not useful ones.
Costumer is a noun. It means someone who makes, sells, and/or rents clothing for parties. It can also mean the person responsible for dressing characters in a live theater or movies.
Customer is a noun. It means a buyer, someone who is surveying the marketplace for things to purchase. It can be used more figuratively to mean someone a speaker has to tolerate or put up with.
The following story uses both words correctly:
Cusick, a high-profile fashion designer, was asked to consult the costumer of Broad Street theater productions. Though he could be brutally honest and hard to work with, the theater found his association with their productions brought them many more customers.