Hissyfit - 10:26 pm on Aug 18, 2000 (gmt 0)

I am fairly new to ecommerce. I have an online fragrance store. I just received an email from a customer who purchased 2 items that said she got her order but only received one of the items. Now, I packed that order myself. I rolled both items up in one long strip of bubble-wrap and know absolutely beyond a doubt that both items were in the box and she is lying. The package was sent by Priority Mail if that makes a difference. I have sent her an email telling her I know I packed both items and asking her if the box had been opened when she received it.
What can I do to avoid having to send this dishonest person a free expensive product?
Thank you.