Obviously they'll be safer in a box, but bubble wrapped inside a padded envelope should be just fine. I received two bottles in a swap that were just thrown in an oversized envelope and shipped. No bubble wrap, padding or anything. Surprisingly they arrived in one piece.
What does become a problem when shipping decants is the pressurized environment of an airplane causing the decants to leak. I have received a few leaky decants, and have not found a remedy for this.