Car dealerships can be shady, especially the smaller used car lot "buy here, pay here, no credit, no problem!" type of spots. If they tacked on a costly extended warranty, you MAY be able to get a refund for most of it, or even all.

Hopefully your state's rules on this are consumer friendly. I know for an absolute fact that in certain states you have a window to notify the warranty company that you changed your mind and don't the coverage anymore. In some states, as long as you cancel in the first couple weeks, they have to give you back every dime. In other places you might only be eligible for a pro-rated refund. I'm assuming that in some shitty fuckin' states they can probably just keep it and you might be screwed.

If they refuse to refund or try using stall tactics, look into which state regulatory agency has jurisdiction over warranty companies and file a complaint.

Hopefully the bulk of the added cost is a warranty. If its shit like "weatherproof undercoating" or some other nonsense then it might not be so easy but you could look into returning the vehicle at that point.

Again, this all comes down to where you live and local regulations, but don't listen to these other posters saying you're boned. Good luck!