You make a strong argument for investing in quality test kits.Almost impossible for me to get a Rx for CP where I live. Can't say that I'd use the eBay stuff.
Something to ponder: the primary thing we do in this hobby is water quality. Everything else is secondary to the water. So great RODI + good quality salt + good quality tools (pumps, heaters, testing equipment) = healthy fish.
I wouldn't buy CP from eBAy either. If my vet writes me an Rx I'll get it from Diamondback Pharmacy as recommended by HumbleFish.
Just looked at the photos posted by Daniel James (https://www.reef2reef.com/threads/proper-dosing-of-coppersafe.309726/page-11) (post #209) and I see what you mean about holding the test vial above the chart. Plan to do tests and post results later today.