thats the main reason I don't like working on A/C systems I bet the evpansion valve was bad he replaced it and the receiver/dryer is replaced anytime the system is opened to air as it's a filter ...but since theres still the orginal compressor,condensor,evaperator,hoses ,hose valves,rubber seals that can fail and usally do since the it did blow cold then stopped it was working fine when it left the shop and after a few weeks either the compressor went since the pressure is high on the low side or some other problem but I would have the shop look at it if it's somthing that they replaced it should be under warranty if not they will let you know what the problem it.