Hi,
I've been using my Credit Card with HTC for the past 3 years or so and I can say with certainty that I have never had any problems or worries as far as security is concerned. Even if your worried about sending this info over the net you can always ring them with the details. I know if I had to remember to send a money order every so often, I'd either forget or leave it too late. AT least with the CC option you can just play and forget it.
ANyway main thing is you've signed up:)
Have fun