A service charge is when you purchase a specific item and also purchase the service of a specialized individual to make the item work for you. The amount due to the individual is called the service charge. Often times there is no additional sales tax attached to the service charge. Since people often have to pay for a part and the service together, it happens often that the sales tax is only administered to the part. There are a lot of states that make it mandatory for service charges to be subject to sales tax, though.