You can always find thing cheaper on the internet. However ordering a part online does not work well with mechanics business and majority of people’s schedules/lives. They want to get a part within an hour or few hours, not a day or few days. Part of the “extra” cost is getting a part delivered the day needed and quickly. There is also a markup on the price for for fair profit of mechanic.
Mail order is for DIY or a not so busy shop.