Man I sure do wish I would have bought an extended warranty. My 13 with 33k miles AC isn't producing like it should. I am dreading taking it in to get looked at. I know it's going to cost me dearly. Anything I should check 1st before I drop it off?
Whether you take it in or do it yourself, you should at least look at the lines for any evidence of leakage (oily residue from the PAG oil that circulates with the refrigerant), and see if the compressor clutch is kicking in with the A/C on. There's a good thread on here about compressor cycling that may or may not apply.
I've used those $20 gauge and can R134a deals a couple times with pretty good success on different vehicles. Not so good on my wife's little 4-cylinder Sentra, it's so tiny a slight overcharge was too much and I had to go buy a set of gauges just to bleed off like 0.14 oz. That said I'll be putting a can of R134a in my '13 SCREW this weekend, it was 109 here yesterday and I know another 12 ounces won't hurt.