Who do you contact when a repair shop?

I had a radiator replaced in my 2000 Ford explorer by a certified mechanics shop. They repressurised the cooling system out wherever the coolant flows through the engine. Well when they did that they caused a leak in the middle of the engine. I paid over 600 for the initial job. Now they're saying 1300+! I don't have money like that laying around for a vehicle. They put their hands on it in the first place. Who can I contact to get this matter resolved?
Please and thank you.

The matter was being resolved, Its not their fault your cooling system was neglected by not following the required service intervals, After the obvious problem was addressed, as in your leaking radiator, the technician did a pressure check to determine if any more leaks were present, apparently there was, this is standard protocol, if it were to fail under the pressure test in the shop it would have also failed out on the road, some mechanics get a bad rap for doing halfass work, would you have you felt any better had you taken it back to them leaking again and complaining they didn't do it right the first time, If it happened at my shop me and the tech that did the work would be having a conversation in my office on why he didn't do a pressure test

Leak was there, now its noticeable. Put a five dollar can of stop leak in it and hope it works long enough to sell it, or fix it right and realize the cost of owning a vehicle.