I don't sell cookies, but my profession is to sell in general. You will always come across customers who will never be content, no matter what you do...
She complains the cookies are dry. Well, of course they are! The are cookies decorated with icing, they need to be dry so as not to affect the icing! Especially shortbread is very dry as a standard, so this is no reason to complain. And I very much doubt that humidity would have had any impact on making them drier than usual 
The cookies didn't match her expactations? Gosh, did she send you a list with them to start with? How are you to fulfil expactations you don't even know? That's also no reason to complain, tastes are different. What one person loves, the next despises.
Do you have reason to believe that something is wrong with your cookies? No? Than stick to them. Offer her 5% refund out of pure good will and tell her you are sorry that she didn't like them. And if she leaves a nasty comment, so be it. A nasty comment among lots of nice will only make your feedback sound more realistic and "real". If someone has 100% positive feedback, that's what makes me suspicious...