Here is how mystery discounts usually work: The more items/money bought or spent, the discount will be bigger. The less you buy, the smaller it is.
I bought $60 of Crowns when the sale was going on, and i got subtracted $6. Yea, not a lot, but it is better then paying $60 or even $80 with no sale! I have one more mystery coupon from my other email account. I will use that when i purchase my Epic Bundle. If you cant add discounts to that, no problem. If you got a mystery coupon for 10% off and your steaming because of it, be thankfull you didnt pay full price on that item!