Wrong Price

A man is thirsty and stops at a convenience store to get a drink. He has a coffee mug which he keeps in his car to do refills because he knows that a cup of coffee is $1.29 if purchased with a cup, but only $1.09 if one brings and reuses his own cup. Twenty cents is twenty cents, after all. When he gets in the store he decides he also wants some potato chips. There are two sizes of chips, which are marked $1.49 and $1.89. He picks the $1.49 bag of chips and brings both items up to the counter. The cashier says, "$1.09 plus $1.49. That will be $2.58."

"Wrong!" declares the man. Why would the man say that this is wrong?