How secure is the Paypal license generation?

When a license generator is created and the ‘Use IPN’ box is checked, an optin form cannot be used to generate licenses.
An access code is synchronized with Paypal, as well as other security checks to deter any fraud IPN messages.
A license is only generated when an IPN is received from Paypal indicating that the funds have cleared from the Paypal transaction.
Before that time, no license will be generated or license emails sent to a possible customer.