It appears under postage preferences and any of your free postage items will have it showing unless you opt out. I tried it on some of my listings for a while but I really don't think that 4 sales over a 6 month period warrants offering it at all. When I was opted in I had to go through any free postage listing that didn't qualify and untick the box within the listing - it just wasn't worth the time it took.

In answer to the OP, I had no issues with the few that we did send.