There's a slight chance that this sort of service may in fact be illegal, or at least in a gray area, depending on who performs the sample draw: https://nppn.info/index.php/blog/view/iggbo-is-illegal
Typically, if you order a test, the person performing the test has to be actually employed by the organization that pays them (ie, cannot be a contractor), and there aren't national-level service providers for things like phlebotomy, because the economics don't really work out yet.
Also, if the provider (MD/NP/PA) service isn't actually doing the test themselves, then I believe they have to provide the option to the patient to choose what service they go to (Lab Corp, Quest, your service or another) because not providing the customer a choice may violate Stark law prohibitions on inter-provider referrals.
I am sure you have considered much of this, but would love to hear what you've found so far.