The classic example that everyone brings up is airlines. If you have 3 random people sitting in the same section of the same airplane, odds are that they paid 2, and maybe 3 different prices for the same exact ride. And if one of them didn't want to stay the weekend before coming back, that one certainly paid more.
If this still bothers you, consider someone at a swap meet, bargaining with everyone who walks up. Each person gets a different price for the same product, depending on how well they negotiate, how you're feeling, etc. It is no different.
A set price is a convenience, no more, no less. And if you have one, and are successful, you'll eventually run into a large company that hires people whose entire job is to change those prices for themselves. Oh sure, they'll wrap it up in nice phrases like "do you support volume discounts" but at its heart it is the same as the swap meet.