I suppose I can take the vouch. I'm pretty sure you can just get 20% off steam funds by buying a steam card off of amazon usuing purse, but let's see if your method is a bit easier.

Update: Vouch delivered. This method will work but it's quite a headache to do and you'll get a 10% bonus at very most. In my opinion purse.io is easier and you get more, but I suppose this is quicker.