Guidance related to Paypal Integration Using Codeigniter
Just to let you know that we've just open-sourced our replacement CodeIgniter PayPal library, called codeigniter-paypal-ipn. (We use this library in production.)
It performs more validation than PayPal_Lib, and it also saves the orders down to your database (using either CI active record or Doctrine).
I hope you find it useful!
There is an existing library for integration with Paypal using CodeIgniter:
I asked the same kind of question here.
And http://www.phpfour.com/blog/2009/02/php-payment-gateway-library-for-paypal-authorizenet-and-2checkout/ is good resource.
Please have a look.