What is learned about the coupon?

M I have a coupon that gives me 20 percent off on one item. I’d like to apply this coupon to the file cabinet, please.
W Actually, the coupon automatically applies to the most expensive item on your bill. I scan it after I’ve scanned all your purchases, and it automatically chooses the item.
M Oh, good. I just wanted to make sure it wouldn’t be applied to the paper or the scissors or anything like that.
W Nothing to worry about. Okay, your coupon gives you $16 off the file cabinet and your total comes to $105.75. How would you like to pay for that?