Hey oh
Pardon me for this, EEEWWWWW. Had to say it lol. I would sugest that you are caught a bit between a rock and a hard place (no pun intended), in that "don't fix it unless its broak" is equally true and untrue for you.
1. Reallly, if you have a formula that is well enjoyed, don't change a thing. No one will be happy at all if you do. My grandmum used to do mince with hard icing, and one year decided that she would just not do the icing and EVERYONE in the family badgered her for 10 years to PLEASE MAKE THAT AGAIN LOL. She did make it again befor she died. She was an awsome baker.
2. How BIG are your shortbreads? Are they the 8" roundes done traditionally patterened?
3. Pack better! Get crafty. Think that they can't flex, or they break. So, a layer of cardboard to lay on, As many cut out layers of cardboard to make walls up the side, and a layer of cardboard for the top. Wrap it in a smooth layer of waxpapper, into your tight carboard box, and then pack that box into a box with foam, or styro chunks or some other damper.
4. If you want to change anything, chnage the size or shape of the shortbreads. My grandmum did flat rounds at home, but she always did 3/4 inch balls for "sent" packages. Formula was the same, taste was the same, and the balls shipped better.