Great

with many possible solutions: My two cents worth of help/advise is,
Our time is valuable and worth some money in return.
Our wood is not free most of the time and worth some money in return.
Our tools cost money and will wear down, this is worth some money in return.
Our paints/finishing cost us money, this is worth some money in return.
Starting to get the picture.
So when a charity donation is asked for, I believe we have the right to discuss the possibility of helping the charity raise some money with one of your pieces. However, to cover these non-recoverable expenses, my position is very simple, your get the piece to auction and I will donate 60% of the final price to your charity and I keep 40% to cover some of my expenses.
hence, they end up with a nice carving to auction off and earn 60 % of final bid while you get at least 40% of value, and it still looks like your making a donation.
If they are not interested in working with you in this manner and be happy to make 60% of your talents, than I would be very cautious in this particular charity group, and would walk away with a very happy answer, "Not Interested at this time".
Good Luck, Coffeeman