USPS lost my package, is there anything I can do about it?

I sold a text book for about $50 through Amazon. The customer wondered why she hasn’t received it yet. Today I received a letter:
“The enclosed was found loose in the mail or damaged by mechanical processing equipment. We sincerely regret the inconvenience caused by this incident. In the course of processing mail, items occasionally become separated from a package and may be lost or sustain damage. Please accept our sincere apologies.”

>_<....REEALLY??? I had delivery confirmation on it but I don’t have insurance policy. I’ve sent books out for years and never had this issue!