Yes the site you buy BlueGUI from is secure.
It isn't PayPal, I chose ShareIT instead because they deal specifically with software.
What happens when you order is that ShareIT process the payment on their secure website (You should be able to see a padlock icon in the bottom-right corner of the browser window to indicate this), and then they send me an email with your name and email address confirming that you have paid, I then send out registration details.
ShareIT also process orders for TheGameCreators and many other small software developers like myself.
BlueGUI Windows Plugin