If they want to figure out if they have enough money to pay for an item before the receipt spits out, they'll certainly have to do the math. What planet do you live on?
And I live on the same planet you do and know of no one who stops to think what the "price plus tax" amount is today as part of their buying pattern. Actually, posting tax inclusive prices would be very helpful. When you see a $429 tax inclusive weekend offer from a resort you can tell immediately what the price of that weekend will be. If we were under the FairTax, you'd actually be paying a cost of LESS THAN the $429 since your effective tax rate would actually reduce the cost from the stated tax inclusive price of $429.
In addition, you wouldn't have to earn, say, $572 (if your effective income tax rate were 25%) to have your "$429" resort weekend. With the FairTax you'd have to earn $429 ... and you'd actually have money left over since your effective tax rate would reduce your cost well below $429.