they take all forms of CC now
Payment
All prices and dollar amounts on this site are in United States dollars. Costco.com accepts the following credit cards: American Express (the preferred credit card of costco.com), VISA, MasterCard, Discover, and the Costco credit card. Some electronic transactions may require extended processing in addition to credit card authorization. You will be notified by a Costco customer service representative if extended processing is required to complete your transaction request through our web site. All credit cards must be valid, authentic and you must be authorized to use them. If the charge is not authorized or accepted, we reserve the right to cancel your order without notice or liability. We reserve the right to stop accepting credit cards from one or more issuers. Your credit card will be charged when the goods are shipped to you.
edit: now that i read it, not sure if that is online only