todo.txt

// To Do List
 - course_settings.php, implement is_cancelled, needs to change the displayed course price back to what it actually is.
 - Implement sending messages to user and/or admin when purchase error is detected. Currently is only a paypal function, where it should be done on the manager layer.
 - Add checks to ensure a user cannot just set the course id in the url and purchase it. They must go through the proper route.
 - Might be a good idea to ensure the course cost in the database, matches with the amount paypal says is being paid. (Currency type makes this possibly quite annoying).
 - When paypal duplicate txn id is detected, report error. There might be something amiss other than paypal double sendign the reply.
 - Translate PayPal payment_date into unix timestamp, instead of current implementation of getting time() once IPN is received.
 - Add checks for when the Paypal subplugin is in sandbox mode. So students dont by courses when the subplugin is in sandbox mode and such.