You can buy Leanpub books using a credit card or your PayPal account. Leanpub accepts Visa and MasterCard only. We do not store your credit card data on our servers. Instead, we send this form securely via SSL to our payment processor, PayPal. Specifically, we use PayPal Website Payments Pro to process credit card purchases through our credit card form. Incidentally, this is why we don't accept American Express: we're Canadian, and in Canada, PayPal Website Payments Pro doesn't take American Express. Sorry.