We've had very bad experiences with them. The first time, years ago, we gave them our credit card number and it was extremely difficult to get them to stop charging it.
Later, we learned that you can purchase gift cards and use them. They will still ask for a credit card . . . just say you don't have one.
Radio Shack carries the gift cards. Since they have filed bankruptcy, I'm sure you can Google and find other retail stores. Or you can order the gift cards online.
The gift cards make it simple, no possibility of them charging to your cc. Another tip . . . they have lower priced plans for basic radio that they won't tell you about unless you ask.