Katrina from WorldNomads.com here. You can buy a policy via WorldNomads.com using your visa credit card (although some visa debit cards wont work, depending on your bank).
For security, we do not store your credit card, so each time you need a new policy or want to buy more cover/extend your trip, you will have to re-enter your credit card details.
If you are having trouble using your visa card, it is a good idea to contact your bank or advise us of any error messages that you see when you try to buy a policy.
You can reach our customer services team here if you have further questions or issues with payment: http://worldnomads.com/contactus.aspx about 5 years ago