I've personally never done it through Steam. I go through the Redfox website since they accept PayPal anyway. Link is https://www.playredfox.com/users/billing/choose_credit
just press "charge" in the Item Shop, the game will open your IE and redirect you to the payment site, where its possible to pay via steam wallet. you cant pay via steam wallet on the website. Also make sure that you are using the steam client.