It seems to me that if a vendor sold something to a customer and they have an adverse reaction, the burden of proof would be on that vendor to prove that their quality control is up to par and to produce a paper trail that proves just that, not for the customer to have the liquid lab tested. After all, a vendor is supposed to be the one making sure their product is safe for consumption, not the customers. Buyer beware of course comes into play, but on something you put in your body, it's a little bit late for the customer to test it once it arrives in a supposedly ready to use form.
The vendor is making their money based on a ready to use, supposedly quality controlled product, at least it is presented to the customer that way, or they wouldn't buy it.
Any vendor that cares about their business would be more than alarmed to hear something like this happened with their
juice and should be more than cooperative with their customer's concerns, especially if more than one person experiences something like this and they know about it.