An upsell occurs when the customer brings their vehicle in for a specific service and the mechanic notices other maintenance or repair needs they offer to the customer.
Not all upsells are a bad thing. The average customer doesn’t (and probably can’t) know everything about their car or truck. It is in the customer’s best interest for the mechanic to note other issues found even if it isn’t why the customer brought the car in. It could be something serious or it could be something that can wait until a more convenient time. A good shop will educate the customer on the vehicle’s condition, help them to prioritize what’s most important, and then work with the customer to schedule repairs.
On the other hand, some businesses use upsells as a major profit source. They offer discounts on other services to attract the customer in hopes of finding other things wrong with the car that they can do at a higher price. See Loss Leader for more information.