Yep, that's where I ended up - including the SPIFF as part of the S/E income and reported on Schedule C (no w2 for owner)  - As I mentioned earlier, I was going back and forth with myself on this, and I ended up including it as s/e income for the same reasoning you used - without the business relationship (and sales of products, etc) there is no SPIFF forthcoming.

As far as underthinking, I do that a lot :).