A diffuser charm is a small vented case with an absorbent pad inside; you add a few drops of whatever fragrance you like and the pad holds it for a week or two. It avoids the plastic-bottle format that clips to a vent.
Because no fragrance is included, there is nothing flammable in the box and no liquid to leak in transit. The case is carved sandalwood with a sliding lid; replacement pads are a standard size. Keep any oil you add away from plastics — most will mark them.