It is not necessary that the function decreasing over a given interval always be negative.
A function f(x) (value) decreases as x increases.
This does not mean that value of f(x) is negative.
It can have positive number as range.
First you add up all the items and the coupon then you will get $24.51. then subtract $50.00 take away $24.51 then you will get $25.49. So when she left the store she had $25.49