DoubleScroll uses the Kagi Shareware registration service. They offer a very quick turnaround and take more payments types than I was able to in the past.
To register, run the Register program which is included with the DoubleScroll package. In there, you will be prompted for some information about yourself as well as payment information. When finished, either mail (or email) your registration to Kagi. They will in turn give you a registration code to enter in the box below. Just enter the number in and you're registered! It's that easy!
