I'm not sure you have to worry about it...if you haven't been having problems reported . Even in the heat of summer the cargo holds of airplanes can get right chilly and a lot of our shipped products make at least part of the trip by air.
You are right Smitty... the cargo holds on most planes are not heated nor cooled... just the ambient temperature... on all cargo planes, you may have a little heat as well as on the acft with engines in the tails, but that is usually just radiant heat as the engines are not near the body of the acft anyway...
In years past we've had people stow away in the cargo holds and nearly freeze to death when the flights reach their assigned altitude... sometimes as high as 35,000 feet.
I've never done anything special on my pens... I always ship them in a nice pen box, then that box is in the USPS priority flat rate box or envelope with a little bubble wrap around the pen box... so far have never had a complaint.