Mail Fraud

I am from Indiana, I own a small business and have a website. I had a lady place an order on my website, insurance on the package is optional, she chose not to insure it, I mailed the package, I have a cash receipt, it does not tell who the package was sent to, just that I mailed a package and paid 9.20 postage. She never recieved the package, or so she says, she has reported me to the Better Business Bureau, and she has contacted the Mail Fraud division of the US post office , I have recieved 3 letters from them, I replied back to all stating, that I mailed the package, that the buyer opted not to insure it, so I should not be held responible. The last letter states that I need to contact the buyer and resolve this matter.The package was shipped to Colorado, in case that matters. The buyer wants a full refund of her money, postage and all. I feel like I should not be held responsible for the mishaps of the US post office. How far can she take this, it wasn't enough money for me to go to federal prision for, but the principle of the matter is, and what I keep coming back to, If she had insured that package, there would not be a problem, so I feel she is to blame. Can she legally charge me with mail fraud? Thank you in advance for all responses.