I am a well-known jewelry designer who ships to approx. 200 stores.  I
have a flagship retail store of my own.  I have been asked by a friend
who is opening a spa in the next town to design a line for her spa. 
So as to not compete with my store, she and I agreed that her
particular design be exclusive--not to be sold anywhere in the state. 
Therefore, I cannot sell it in my store either.  The question is how
do I charge for the exclusivity and one-time design fee?

Have you considered a royalty fee on each item sold ?