“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. Note that in some cases, Steam will be unable to give refunds for some DLC (for example, if the DLC irreversibly levels up a game character). In this case you will be warned when purchasing such DLCs.”
Point is, different from games that you can play for 2 hours or 14 days, after this DLC is applied to your ingame account you will be unable to refund it.
Edit: DLC’s like map packs or new campaigns for strategy games can be refunded normally most of the time.