If they are going to be offended for having to pay a discounted rate for cookies that you can’t give them for free due to fees and resources than that’s their problem.
You tell them how much to pay, how they react is up to them. One thing I have just began learning is I can’t control peoples emotions even if I want them to be ok with something and in my mind I believe there is good reason; the simple fact is not everyone thinks alike. So do what you can but after that it’s their fault if they make a big deal over some cookies.
^ this is the right answer, I assume paintings turned from a hobby to a business at some point. It may have been fine to tell your friends that you could or could not deliver because they have an established relationship with you, but you can’t put that ambiguity on your paying customers.