(Disclosure: This site contains product affiliate links. We may receive a commission if you make a purchase after clicking on one of these links. Read our affiliate disclosure for more info.)
Bundle Stars has a Strike Lucky Sale promo running. From today until June 30th whenever you spend $8 / €8 / £6 on bundles or store purchases you will receive a mystery gift, which can be Steam games or even a Steam Wallet credit.
You can see a banner for the promo on the Bundle Stars Store page.