That actually isn't fair. Back before I was an accountant with an adding machine, I could do all sorts of math in my head, fairly complex. Once I started using the 10-key, calculators, and such, which I had to do so I'd have the ticker tape to attach to my reports as "proof" for superiors, I slowly lost the the ability to do math in my head, but my fingers now can do it super fast. So is it stupidity or is it simply a reliance on machines that never required one to have the skill to do math in your head quickly. Even in math class, pre-calculator, you had pencil and paper, very few people who are not regularly doing it, can figure change. Nowadays, almost no one is required to do that unless and until the machines go down.
Anyway, in the end you're being entirely unfair to judge current education levels based on being able to figure taxes and change in their heads. Not to mention, anymore with inventory so connected to the cash registers, companies simply will refuse to let anyone do what you're suggesting but will instead stop all sales til the registers are back up.