Hi Dmos,
What you describe is a classic Quality Management dilemma, one which small and large businesses alike grapple with everyday.
You are lucky in a way that you are a perfectionist crafts person who is very close to your customer and is so passionate about your business.
If I were you I wouldnt get too hung up about how cheeky your customer is (this might be true but it wont get you or your customers very far!)
Step1. Ensure that immediate customer is happy.
If your customer was a longstanding good customer I would offer her a free cake.
Step2. Identify what went wrong in this instance.
Enlist the help of the customer (Make them work for their "free" cake!).
This step is not as easy as it seems but it is more than worthwhile in the long term.
Step3. Take steps to ensure that this does not happen again.
Some other posters have made some excellent suggestions (some form the confectionary trade) as to how this might be done.
Hi dmos 87. My sister is a professional confectioner in Montreal. Over the years she has developed a system to avoid such complications. An order form, detailing flavouring, layering, frosting, colouring, designs and size. She also kept a photo portfolio of previous designs. If someone leaves the ingredients open, she went with the standard Lemon or Vanilla sponge. Hope this helps and good luck. A price hike may also be in order. You seem to be on the low side.
Step4. Encourage Customer Complaints and Comments.
Seperate out the spurious ones and act to address the issues raised by the genuine comments/complaints.
then return to step 1!