You didn't say that he fixed the leak, just that he added Puron. If he didn't find and fix the leak, he will be back to do it all over again. House air conditioning systems, refrigerators, and home freezers are all completely sealed and should not ever need re-charging in their lifetime. If re-charging is necessary, then there is a leak that should be fixed before any more refrigerent is added. Many of these service guys make a lot of money from customers with slow leaks in their units. It guarantees that they'll be back in a year or so. Does the service contract that you bought include parts and refrigerant. I'm willing to bet that it doesn't, only cleaning and filter changing.
Charley