It seems odd but it works. I only suggest the prepaid option if you are in a not legal area. Just to protect yourself. If you're in a lucky state or country I wouldn't worry.
The way the ordering works:
Place order on website
Text them that you have ordered and include your order #
They text you back a link to payment/invoice
From there you can use CC/Paypal/Whatever
They confirm payment and you get a receipt.
Mine was pretty much instant since it was middle of the day operating hours for them. If you order late they may get back to you in the morning.