http://ow.ly/cgLMT. Big thanks to Alyson Stanfield, of the Art Biz Coach for sharing this information! Of course this is not specific to pencil portrait shipping, but a lot of the advice applies, and/or will give you something to think about!
Also check out:
3 Great things to include with your outbound orders
Mailing your pencil portraits
Repeat after me...no more round mailing tubes!