I tend to come across pretty strong and posts like yours are so common and usually newcomers or folks doing anything possible to avoid a QT. I wasn't trying to chastise; that's just my nature. But you explained most of my comments. But IMO & IME; there just is no substitute for using your QT, in your house, and properly quarantining all of your fish. I can think of a zillion ways the porc at the LFS could have been infected with ich. Little kids wandering fingers, I've seen (many times) that a lfs does not have separate nets of specimen containers for all fish, I've seen fish fed from a paper cup and the employee dips the cup into tanks to get more water in the food cup, etc., etc,. I will never trust any LFS to substitute for my own quarantine protocol. FWIW; I have about 1000 gals of DTs in my house and haven't seen ich in any of them in about 20 years. I buy all my fish online and always will. DD, possibly the best source of fish on the planet, strongly suggests you quarantine their fish.
I once had a video, made by a Realtor, showing the employees of a huge, well known east coast LFS opening for the day. You could easily spot several practices that could move forms of ich anywhere. Things only a hobbyist would notice. BTW; Brute plastic containers are fine for tank-transfer and Craigslist is full of cheap stuff. But I would use Cupramine as I have on all my puffers; but at a lower dosage and longer duration than what's on the bottle. .35-.40 ppm for 6 weeks and introduced over about a week.