Honestly, the best freebie I took part in as a buyer was offered BEFORE checkout. I thought that was a great idea and once I have something I can offer like that, I might take that approach.
The seller put in her shop announcements (that many people forget to check) - something like: "This month only: 20 free tags with purchase! Just mention it at checkout what size/colors you'd like." (Obviously more descriptive than that.)
So yes, it's up to the buyer to see it and put it in their cart notes, but this way, I was able to see another product the seller offered and had to put in minimal effort. Which, let's be honest, buyers need minimal-effort opportunities! :)
-Aurora