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What makes it (possibly) self employment is that the money is not coming from your employer.  It also appears that the product (warranty) that you're selling is not your primary product and from a different vendor than you primary product. As such, it may not qualify as a SPIFF. It's a gray area. If you have CPAs familiar with the arrangement telling you to report it on Schedule C, they're probably right.
The IRS is aggressive in looking into 1099-Misc i(box 7) income that is not reported on Schedule C. Most SPIFFs go in box 3 of the 1099-Misc.