If you've done the diagnosis from the pushbutton, do your self a favor, buy a set of R134a guages on eBay (~$50 or so). Then, with a termometer, you've got all the tools you need to charge the low freon yourself. Do a search here and you'll find plenty of information about the correct high and low side pressures and ambient/vent temperatures.
R134a sells for 4.99 for a 12oz can here
http://store.lenzdist.com/