The normal method of using PHP-SecureArea is to use PayPal subscriptions which is a recurring payment. However, you can also create a button which is a normal PayPal 'Buy Now' button and not a recurring subscription button. You could use this when you want to create a one-off access to the secure area, for example you may want customers to access the secure area but do not need it to expire.
In fact, you can still expire access to the secure area, but you must have the CRON job running for that to happen.
You may also want to use this method when you do not want people to sign up for a PayPal subscription. To use a PayPal subscription you have to have a PayPal account. To use a PayPal 'Buy Now' button you can pay by credit card without a PayPal account.