The merchant will usually not verify that you used a Discover card. If you use the Discover referral link, the merchant will pay Discover and Discover will pass it on to you. However, remember that if there is any problem (cookies don't work correctly, for example) and you didn't use Discover, you won't be able to do anything. In my experience, around 1 time in 4 there is a problem and I have to request the cashback manually.