2006 Chevy HHR - A/C only works at high rpm's - is it just coolant needed?

My girlfriend owns a 2006 Chevy HHR (probably irrelevant). The A/C in her car only gets cold when we drive at higher speeds. At slower speeds and idle and the air gets really warm.

I'm hoping someone who knows something about A/C can tell me - is this a simple coolant issue? I see they sell kits now that you can recharge the A/C coolant yourself. I would prefer this option to paying a mechanic 3x as much to do the same thing. But it's a waste of money if it's the compressor or condenser.

Thoughts anyone? Is there some test or better way to be sure what's going on?