When someone comments that they want to earn a mascara, I first take a glimpse at their wall. Again, my goal is to avoid chronic sharers for this affiliate program (doesn't mean I can't ask them to host an online party though!).
Then I message them something really simple:
“Hi Susy! I saw your comment on my mascara post -- would you like to get the deets on how to earn a free mascara?”
Notice I say EARN — I want it to be REALLY clear that they are earning this, not being gifted it.