There has been some talk in the forums recently about including a "freebie" in with the purchase a buyer has made. I thought I'd include my 2 cents!
I think free gifts are tricky. I've received several with things I've ordered, both on Etsy and through other sites, and here's what I know as a buyer: There's always a nice little "aw...!" moment when I find a free gift, but then I think, what am I going to do with this? Often the gift is not related to my order at all, but something in the same selling genre, and while it is neat, it doesn't serve much purpose. This is especially true in my genre of Jewelry. Unless the gift is for a repeat buyer you know, or surprise matching earrings for a necklace ordered, you don't really know the buyer's style. Perhaps he/she is buying a gift and so the item is not indicative of what he/she would ever actually wear.
Also keep in mind never to send food or something overly scented (i.e. something that makes everything in the package smell like that) as a gift. In today's world, there are so many allergies that it's not worth the risk. Even if someone ordered food from you, that doesn't mean he/she can eat everything in your shop.
The best free gift I've received was when I ordered one item in several different colors/patterns/etc. and my gift was an additional item of what I had ordered but in a color/pattern that I hadn't.
My closing thought? If you want to give a free gift, that's great, but make it directly related to what the person actually ordered, or it may end up in the trash or Goodwill bin.