PDCalendar Free is a free download and does not need to be
purchased.
For the Pro-features a license is required.
At the moment I don't have an automated process for purchasing
PDCalendar.
PDCalendar Pro is registered per second-level domain (like
mydomain.com or yourdomain.co.uk). The key will be valid for all
subdomains for that (like me.mydomain.com).
1 license costs 50 euro. Payment must be done through
Paypal.
For now, if you want to purchase PDCalendar Pro, contact me by
mail: peter [at] dijksterhuis [dot] nu