A few tips for you:
A small box, a padded envelope, or a small box inside a padded envelope should all be fine. If you're opting for a padded envelope alone, bubble wrap the hat.
If you're going for just a small box, make sure it's not TOO small. It's a bad time of year to be posting things, and anything that's too small could get lost.
Make sure you put the accessories inside a ziplock bag whatever way you send it, especially since you're sending a tiny barrette. I actually had a padded envelope burst along a seam once. The accessories were not contained inside anything, and while I got most of what I paid for, several items were missing. We also found a few random pony shoes on the lawn and the driveway from that lot. Had everything been inside a baggie, nothing would have fallen out the hole, and I would've gotten everything. Small items only need a tiny puncture to escape!
Reinforce any joints or seams with tape.
This time of year, insist on insurance and a tracking number.
Good luck with your sales!