Once you enter your email address and enroll, you will be able to see their entire site- products/prices. They have specific pymt methods they accept.. one being Visa. Once you decide on what you are ordering and go to pay, you would need to select a pymt method and enter in all your cc info, as you would for any type of internet purchase. When you make any other purchases online, your billing address needs to be associated with your credit card. If you don't want to use a credit or debit card that you already have, you could consider something like a green dot Visa card. You purchase a temporary card at locations they are available at, like CVS, etc, then set up an account to obtain a permanent card that will be mailed to you. The permanent cards can be used for international purchases and have your name on them and are also associated with your billing address. With this option, you can load only a certain amount of money on it at a time. You can look up the green dot Visa website and review all the specific information like fees, terms and conditions, etc.
EDIT: If you have read through their recent reviews, you can see they have been a very reputable and reliable IOP. I have not read about, or personally ever had any issues using my credit card on their site. No additional charges, etc- whatever the total amount an order ends up being, is exactly what is charged on my credit card. Although, using an alternative card such as green dot Visa, etc is always something to consider, you can never be too careful. There are never any guarantees with any vendor that you use, no matter how long you have used them for.