flexi cart - A comprehensive shopping cart library for CodeIgniter

[eluser]haseydesign[/eluser]
@Matthew Diggins

Good to hear your liking flexi cart!
Regarding your question, I've not used PayPal WPS before so I'm a little unsure of what you're exactly trying to do.

I'm presuming you're sending a set of data off to PayPal, which then in turn returns data to your application which you then wish to use to confirm whether your payment transaction has been successful.
Within the data that you send to PayPal, you wish to send an order identifier which will then be sent back to you to identify the order that the transaction corresponds to.
Is my understanding correct?

If so, then yes you could use the cart id via cart_data_id(), but why not use the auto generated order number instead? - order_number()
The cart id is not automatically associated with the completed order, whilst the order number is, meaning you can link the transaction with the order much easier.