Do you have to tip for free services?

This weekend I'm going to my salon to get some waxing done, and while I'm there I'll also be getting a free facial (and goody bag) that I won on their Facebook page, when they had a little contest a few days ago.

I always tip the esthetician who does my waxing 20% because she always does a great job. I've never had a facial at my salon, but I'm sure it'll be great, like all their services. My question is, do I tip the esthetician for the facial, even though it was free? It's a $150 value, so a 20% tip would be $30...which is kind of a lot. I don't want to come across as a cheap, crappy customer, but at the same time it seems kind of odd to have to pay $30 for something that was supposed to be free that I won in a contest.

Just not sure what the etiquette here is - please enlighten me!