take it to a different AC shop, cause what they told you was BS. The system has a leak somewhere and it needs to be found and repaired. You need someone with the detection equipment (a special sniffer they use these days, old way was to dye test....takes too long and is not as good for small leaks). I doubt you need a compressor, just find the leak and repair...could be many things, some are fairly inexpensive (bad hose fitting), other could be mucho bucks (like a corroded condensor).