I have also used Paypal for business purposes, and I have to say, I far prefer it to the US mail. You can put a stamp on something and send it into outer space, hoping that it reaches its destination, or you can press a key and know that your payment was received instantly. Just another 2 cents.
However, you should note that your official membership response from the CoS will NOT be instantaneous.
Contemporary culture is generally disposable. People are used to instant gratification these days, and want everything yesterday. A wise man once said to me that...' we want to put the brakes on all of that'.
Membership in the CoS is not an impulse buy. The CoS wants people to think about what they're doing, and take it seriously. Your membership will be handled, but it will be handled slowly. If you want to get your decoder ring and start going to grotto meetings right away, you'll be disappointed. If you are a Satanist, you will live as you have always lived, and enjoy your membership as one of life's little pleasures when it arrives and thereafter.