How is this legal under contract law?

They extend an offer, you agree the price, you agree to buy and they sell. But they're suggesting that you buy, and consume the product, before agreeing a price?

Is it just that they don't know what's happening at the doctors? ie, they could offer a base price of $X per 15 minute appointment; $Y for writing a prescription; $Z for referring on for other tests?

Or can they not even tell you how much a basic 15 minute consultation would cost?