Acccording to a Topps representative I spoke to, you can send in a NPN request per NPN address, not per product. For example, let's take 2006 Topps baseball... you'd be able to send requests for:
2006 Topps Jumbos
2006 Topps Hobby
2006 Topps Retail
2006 Topps Rack Packs
Each had different NPN addresses to send to... probably because they have some different inserts (hobby vs retail) and different seeding of chase cards (for example an autograph in retail was much harder to pull than in hobby)....
Now, I admit I had this phone conversation last year, but I doubt things have changed....
Thanks, Michael. That's exactly the kind of information I was looking for.