Views Integration and PHP code for: getting nid for node that user wants to publish once i know order number)

User pays for a product. Earlier user saves a node (for which I need to buy the product). I'd like to get the nid for this node. Especially i need it on Checkout Complete page.

I presume references used somewhere that I can use for this to achieve. I'm thinking of a php code here.

Or views integration (i'm thinking of a contextual filter to connect a line item and the node that has been bought).

Any help is appreciated.

Thanks