Allows site administrators to accept payments using PayPal payment system.
In order to use the IPN feature of PayPal API, you should have the "Business" account on PayPal and set appropriate options on paypal.com.
Notification URL: http://{YOUR_SUBRION_SITE_URL}/ipn/paypal/
(say, http://example.com/ipn/paypal/ for EXAMPLE.COM domain, respectively)
IPN Messages: "Receive IPN messages (Enabled)" option
NOTE: Some of these items may be missing if the type of your PayPal account differs from "Business".
Please refer to PayPal documentation.