Why did you pay IQ tax?

Once I bought something in a shopping mall, and then the cashier told me that I could draw a lottery with an invoice. The lucky draw was at a jade counter, and I won the first prize (the first prize means I can buy their jade at a discount), and then the salesman was surprised and praised me for how lucky I was. Then I happily won the first prize and spent nearly 800 yuan. Later, I felt really stupid.