Tax question on expenses

I apologize if this has been asked before, I reviewed the tax post but didn't see anything about this. Let's say I pay for a $20 purchase to get $50 in credit back (like a Swagbucks offer). Could I write the $20 purchase off as an expense or deduct the $20 from my profits (so I only "earned" $30)?

Just a bit confused and not sure if I should be tracking the things I buy to earn money. TIA