I would talk to someone at USPS before you do anything. I tried calling the 1800 number before, and after like 15 minutes of screaming at the prompt service I gave up and visited my local post office the next day. Make sure you have sufficient proof that the customer never received the item after it was delivered. I know you said she was a sweetheart, but still, cover your assets. If USPS confirms it was delivered, file a theft claim. IMO, it's not your fault that your customer's things were stolen. But, if I were in your situation, i'd probably end up remaking the thing anyway....
This is a sucky situation, I hope you find something that will work for you and the customer!