Oh I’ve just bought an aircon service on groupon at ats for £30 regas & antibac fwiw.
As for service Inc gassing the system they will just drain your perfectly good gas out (R134a or R1234Yf price much higher if its this one as newer) assuming its working, mix it into their system with other gas taken from other cars & their wholesale, vacuum the system down so any moisture boils off, then if the vacuum holds they’ll has it up again with whatever gas they’ve accumulated between wholesale & scavenged from other vehicles.
If your aircon is plenty cold enough I’d leave well enough alone. If it ain’t broke & all that….