It is probably not a good idea to save your payment information for future purchases online. You have probably heard about data breaches where thieves steal millions of credit card records, even from big name companies. Any site where you store your credit card information will have it on their server. If they get hacked, your information could be compromised.

More information: http://www.usatoday.com/tech/news/computersecurity/infotheft/2008-08-05-...