A $5 discount barely registers, but a $5 shipping fee can sour a whole $40 basket.
The fee arrives as its own charge, so the shopper feels punished right before the final total. Behavioral economist Dan Ariely calls this the pain of paying: a visible fee hurts more than an equal discount feels good.
Amazon's Prime turned delivery from a bill into a benefit. The totals can match — free shipping just removes the sting.