Some of us are better at multiplication than others. There are some equations that have been etched in my mind (6x6=36) and many more that I still need to calculate.
Small purchases at this time of year add up... the $5 purchases don't feel like much, until they add up to more than you were expecting to pay.
Here are some good numbers to etch (or re-etch) into your brain.
$15 x 4 = $60
$35 x 5 = $175 ($35 - $40 is a common price point for gifts)
$9 x 4 = $36
Having these multiples in your mind helps you make conscious choices in the moment - so there are no surprises at the register.