It's important to remember that this is a Trial. Customers have the unique opportunity of trying this product before paying for the product itself.
Customers still have to pay for the shipping of the product, which is why a credit card is required for purchase. If the customer decides to keep the product after the trial period has expired then they will be billed, via the same credit card, the full purchase price ($79.99-$105) for the product they have decided to keep. After 30 days customers are automatically enrolled in the monthly membership program.