As Luke with Chilihost said the payment processors have script to add
and remove users and other people have already spent years developing
log in systems of various types for you to use. That's something you'll
notice about Linux/FreeBSD etc. servers. With servers it's the reverse
of the desktop - all the software is already available for POSIX systems
such as Linux and FreeBSD. You don't have to write everything from scratch
like you did with Windows. I'm sure Luke will tell you about a log in and
protection system called Strongbox if it seems to fit your needs.