Why didn’t the man believe his eyes?

W: May I help you, sir?
M: Er, I’m wondering how much this book is.
W: There’s a price tag on the back page of each book. Have you seen it?
M: Yes, I saw it. But I just can’t believe my eyes. Is it really ＄65?
W: Well, I’ll check it through the scanner…Yes, it’s ＄65.
M: Wow! That’s a lot of money for a book.
W: Yes. Medical texts are usually very expensive.
M: Well, I regret choosing medicine as my major now. If I knew this earlier, I might choose to learn mathematics or something else. W : Really?
M: No, not really. I’m joking. I love medicine and hate mathematics. Although this book is quite expensive, I have to have it.
W: How would you like to pay for it, in cash or by credit card?
M: In cash. W: That’ll be ＄ 70.36.
M: But you said it was ＄ 65.
W: Well, with tax, it comes to ＄ 70.36, sir.
M: There’s a tax even on books?
W: I’m afraid so.
M: All right, here are two fifties.
W: Here’s your change, sir. It’s ＄29.64.
M: Thanks.
W: Have a nice day.