You will need to check the bottle of Copper medication and find out if it's chelated or not, then make sure the copper test kit matches. If your med is unchelated and you use a copper test kit for chelated, you aren't going to get an accurate reading, nor will the store.
Did I read this properly where you said the fish went back to the store and 3 months later it still had ich? If that is the case, there is a good chance that what the fish are suffering with is not ich. There are other diseases that look similar to ich, but are not ich. I have never seen a fish survive 3 months with ich... the parasites take over before then and it kills the fish. If the fish didn't have ich and you treated it with copper or another ich medication, this also could have killed the fish.
Please never buy a fish that doesn't appear healthy. It's extremely important to look a fish over very very well before deciding to take it home, and then it should spend at least 2 - 3 wks in quarantine before considering putting it into a main tank.