Get a script for the required medication from your doctor, or an online doctor service like GP2U Telehealth;
Send that script, the authorisation agreement, and your payment to the Buyers Club;
Get your tested medication in the mail ready to use.
It's understood that the payment will be a receipt of some kind that can be emailed, as can the authorisation, but I'm not sure about the prescription. Pharmacists are very particular about these, so I thought to ask about whether the Club needs the original or whether a scanned/photographed copy would be adequate.
Better to ask now than waste time later, so please excuse me if I've missed the obvious answer somewhere.