I purchased some once. They require a valid credit card or debit card.
The way the scam works is when you make the initial purchase you agree to a subscription plan.
They have your financial information and 30 days after the initial purchase you will receive an email informing you of the next shipment.
The funds for the shipment have already been withdrawn from your account.

I specifically avoid the 'snake-oil' sellers. If a drug is good for me then the NHS will supply it - no comeback for those selling false hopes.
Last week they were telling me that a few herbs will CURE COPD!