With IPN you session won't be available in the post back. The easiest solution would be to store them comma-separated and pass them to paypal in the "custom" field. Then you can retrieve them from that field to update the database.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.