Whoa! I am totally shocked... here's what I've found:
It is illegal to posses drug paraphernalia - in my research, where I used to live in the US, it is a petty misdemeanor which basically means it's a petty offense, doesn't constitute a crime, and each county in the state has their own regulations about how this is punishable - some have certain fine limits, etc.
So, I checked state law to see the definition of drug paraphernalia, and it includes anything that aids in "testing the strength, effectiveness, or purity of a controlled substance" ...controlled substance = drug except for alcohol & tobacco.
This makes me angry. I don't want to get started on a "give it to the man" rant but man... how clearly the laws spell out the truth, right?
Overall, based on my quick research, I suppose the maximum penalty you could get is just a few hundred dollars, and I suppose you're protected in that they're not allowed to seize this stuff unless they have a warrant or it's in plain view. I suppose monitoring a website's traffic, receiving a warrant from a judge to go after someone based on a test kit order is all not worth it for the overall value and probably wouldn't happen unless you're ordering large amounts of test kits or are suspected for supplying, producing, etc. BUT STILL... makes me very angry.