It seems so simple on the surface: A customer fills up on gas, purchases some “fuel” for the road—a pack of cigarettes, a bottle of water—swipes a credit card and then heads down the highway. Behind the scenes, however, the systems processing this transaction are going into overdrive as they sort out the numerous tax rules, rates and requirements surrounding this one seemingly straightforward transfer of goods. When a purchase involves both excise tax and sales tax, it’s in fact anything but simple.