That might not be enough to actually reject a charge unless you have a card that specifically will not charge an amount if there isn't enough on the card. I just took a look at their Terms & Conditions out of curiousity, and you might want to as well.
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Paying By Credit Card
For your convenience, when you place your first order and become an official member, we will save your credit or debit card information and use it for all future shipments and charges which will automatically be charged to your saved card on the 27th of each month, unless you notify us. If your payment is unsuccessful on the 27th of the month, we will continue to attempt to process payment for your monthly box at our discretion. We accept the following credit cards at this time: American Express, VISA, and MasterCard.
For your convenience and continuous membership benefits as a client, if your payment method reaches its expiration date and you do not edit your credit card information or cancel your account, you authorize us to continue billing that credit card on file including extending the expiration date until we are notified by you or the credit card company that the account is no longer valid. We encourage you to constantly update your payment method information or cancel your account should you wish to discontinue your membership with us."
And then there's the part regarding chargebacks:
"You must provide and keep us up to date with accurate member information, including your name, address, credit card number and expiration dates. You agree that we may update your information with information your bank or credit card issuer may supply, or other information available to us. All such personal information is subject to the
Privacy Policy, incorporated into this Agreement. We are not responsible for any fees or charges that your bank or credit card issuer may apply.
If your credit card issuer reverses a charge to your credit card, we may bill your account directly and seek payment by another method including a mailed statement."