I'm surprised that no one was able to answer that. When I was taking Psychology class in college, the professor told us people somehow tend to ignore the digits after the decimal point. So that psychologically, 9.99 looks as if it's only $9 something, even though the true difference is only 1 penny from $10. So if you buy a bunch of stuff all ending with .99, then 10 items will be $10 difference. 終究的目的是為了讓顧客多消費。As for why 黑人喜歡喝藍顏色的酒,我特地去問了一個老黑同事，他也不知道。