Here’s the response I got from Steam:
Thanks for reaching out to us about this.
In-game purchases are non-refundable per publisher request. If we were to refund one of those purchases the third party would likely restrict or ban your game account, which we definitely don’t want to happen.
As it does appear that the content purchased in-game is likely only designed to be purchased once you’ll need to contact Warhammer Vermintide 2 support to explain the situation and ask if they can either exchange one of the purchases for other in-game content or confirm that one of the transactions can be refunded without any negative action being taken against your game account.
Please provide them with these two transaction numbers: 2712832256641498870 & 2712832256641500345 and ask them to specify which transaction can be refunded.
You can reach Warhammer: Vermintide 2 support by using the information from the Official Support section here.
Once you have their response please send us full-screen images of your entire correspondence with them and we can go from there.