Cant I put them all in my largest expense category? or across a couple categories?
Expenses go into whatever category applies to a specific expense - which it what I assume you've always done. The cash back just gets included in the general business income. I would expect that since the cash back is a traceable transaction, that's the way you'd need to do it to avoid raising eyebrows at the IRS. Especially if you ever get a tax reporting document (such as a 1099-MISC) for the cash back. But I think chances of that are rather negligible.
