Here’s an article from Coupons in the News about a lawsuit involving Murphy USA and Kit Kat.  You can read the entire article here, but my takeaway is the following quote:

“Such cases frequently occur when coupons are involved. In all but a few states, sales tax is calculated based on the total before any manufacturer’s coupons are used, but after any store coupons are used. When coupons aren’t involved but an item is on sale, as in the Murphy case, the same principle applies based on who’s funding the discount. If it’s a manufacturer incentive, sales tax is assessed before the discount is applied. But if it’s a store discount – as the plaintiffs in this case argue the Kit Kat sale was – sales tax should only be assessed on the final selling price.”

How does a regular customer know which incentive is funded by the store and which is funded by the manufacturer?