Ok Here is the little bit of advise I wish I was given over 20 years ago....don't listen to what any LFS tell you...even if it seems their intentions are good..take responsibility for your own system and QT your own livestock.
All it takes is a hand/net from a shop hand or customer even to go from one non copper treated and infected tank to another to spread white spot. I would not assume the LFS monitors the copper level daily to ensure the levels do not drop below the point where it cannot zap the free swimming stage of the parasite....bottom line is if you quarantine your own livestock and manage your risk as best as possible your success will increase dramatically.
9/10 the fish which by the way are known as white spot magnets came in with the parasite if your other fish have shown no signs of infection before adding the new fish, I don't think the parasite was in your system before waiting to pounce on an unsuspecting newcomer. My humble opinion.....