If you purchased an item directly from the trading floor (such as Mr. Fosters alternate jacket and tie combo) or a USB you can refund it within the following rules:
DLC purchased from the Steam store is refundable within fourteen days of purchase, and if the underlying title has been played for less than two hours since the DLC was purchased, so long as the DLC has not been consumed, modified or transferred. Please note that in some cases, Steam will be unable to give refunds for some third party DLC (for example, if the DLC irreversibly levels up a game character). These exceptions will be clearly marked as nonrefundable on the Store page prior to purchase.
To do so log into your steam account here: https://store.steampowered.com/account/
And navigate to the "View Purchase History" link located near the top center of the page under the Store & Purchase History section. From this page you can select the transaction and request a refund within those 2 hours or until the item has been consumed or used (such as a key) or changed or traded to other users in anyway.