In the past, programmers have turned up ways to remotely view cookies in a browser. This is a problem with the browser, not the cookies. This is a very good reason to make sure you are running a current web browser. Realistically, it is about as safe to use a credit card over the Internet as it would be to use one over the telephone. Data moves across the Internet in clear text.
Encryption makes reading intercepted data extremely difficult.
SSL is designed to allow encrypted, authenticated communication across the Internet. Be sure the site you do business with not only has security but also actually uses it. More on certificates and SSL security can be found at. Many networks prohibit sending such mail traffic in an acceptable use or security policy. This is much more difficult than it sounds.