There is over a billion transmission a day on the net for some one to target you and get your credit card number is a billion to one shot.
If you where a hacker and you could get this off the net would you waste time with some ones credit card that was maxed out or would you be working on banks and kmart where it is at least worth you while to risk getting caught.
if you are not at a secure site then is it a site you trust? (not satan's hacker site) then you are still OK you walk into a restraint in a town you have never been to, you pay with a credit card and you do not worry that the girl is not going to give your number to her drug dealing boyfriend so it is the same with the net.
If you are worried that some kid will get it by accident then just email half the number and tell them the other half is in the next email. The odds of some kid getting both with the millions of transmission that are happen at once well...... you get the message
There is only one truly safe way to do business PGP and public keys read about it here.
Pretty Good Privacy Version 2.6.2 or pgp as it is known is a good place to start, you can get it at all the soft ware places I list.