--Update: $30 minimum purchase now required--
So, it's not exactly a hidden offer. STP has a referral program that says they'll give your friends $10 to spend at the store and once they place their first order, STP will send you a $10 gift card.
I'm struggling a bit right now (college expenses are outrageous these days) and could use some new stuff (shirts, socks, shoes, winter boots). So, if you'd like $10 to spend at the store and help me out at the same time, message me the email account you'd like to use (or post it here) and I will send you the offer.
Also, in case you don't know STP, offers free shipping if you get their riddles right. I have the latest answer if you'd like that... saves another $6-10.