Problem is is that there may be a ... problem. A/C systems are sealed units so when you notice a sudden drop in performance it is NOT because it just needs to be filled up like a tank it means there is a leak. Yes the system needs an evac but truthfully you may just be throwing money away by dumping in more 134a.
Have the system drawn to a vacuum and refilled with dye impregnated 134a (Or grab 2 12 Oz cans with dye in them) and run the system either until the system starts to fail again or for a few hours and check everywhere you can with a UV light. Chances are it will turn out to be an evaporator core which is a bummer (Been there done that) and you will not SEE the leak but can certainly smell it or feel it by sticking your finger up in the HVAC box where the hose connects to the fan for cooling the motor. .
It is very rare that a system just needs to be topped off and is fine for ever. It may get you thru the end of the season depending on the leak. Oh and try to avoid those "Stop leak" quick fill cans. They will add for some more problems later down the road.