An upsell can also be used as a deceptive practice. For example, you reserved a mid-size car through a rental car company. You get there and they tell you they are out of mid-size cars and you'll have to pay $X more for a full size car. In fact, if your confirmation says you reserved a mid-size car and they are out of them, they must give you the larger car at the same price as your confirmation states.
Sometimes, they even use this practice when they are not out of the size you reserved, but rather are deceiving you into believing so in order to charge you a higher price, of which the agent gets a commission.
This has been written about in the travel media and certainly can happen.