If you've been at a big box store Customer Service Returns counter lately, you've probably seen big signs that say "No Returns On Any Halloween-related Items." Even if it's not opened, most of these stores won't let you return a costume. Even costume exchanges aren't allowed I presume.
Different stores have different policies on what they will or won't accept back. If you're like me, and most other shoppers, you never read that Return Policy sign until you're standing in line at the Refund counter. A lot of unpleasant surprises happen while in that line. Is there anything more upsetting that realizing you can't even get an in-store credit for some expensive item you bought and then had to return because it didn't fit or didn't match or you just changed your mind?
Make it easy on yourself. Visit Costume-Returns.com. They have lists posted of what can be returned to the store and what can't. Actually, the website is a resource with some good advice about costume purchases. Read it before you buy for a costume party or Halloween event. They can help you get the costume that's right for you, and it can be one that's timeless. Some costumes can be worn over the years because they never really go out of style.
The Internet can help you make the most of your spending dollar in even when it comes to buying fancy dress costumes.