Needs a freon license, not all regular mechanics have it, but not hard to find one. But usually, if its low freon, then it's gotta leak somewhere because the heat exchange system is a closed loop. So, if its low, you need a pressure test to see where, or sometimes you can see it with colored gas - a test they run on the compressor.
So, a A/C is not cheap, but get your TOTAL costs to fix it first, not just the Freon cost, then find out a switch, then a coil, then x y z. Sometimes it's less expensive for a new one, and then of course you have a new one and not a fixed one that might break again after you sink those bucks in.
Not for one second recommending a new A/C - but just that you look at all the costs before you decide ...