Instruct: Classify the given review into positive or negative sentiment.
Query: What is the customer service ?

The book has not CDs, and I have to bough CD from Internet. So the price of used book + CD equal to new book. I emailed the book seller, they told me they would return me 10% of the book (1/2 of CD price), but now I haven't received the cash back. It is cheating 2 times.