Word Study
sales tax
CIDE DICTIONARY
sales tax, n. [Sales + tax.].
a tax imposed upon the retail sale of goods or the sale of services, usually collected by the seller at the time of purchase; -- it is typically calculated as a percentage of the price of the object sold, being commonly from 3% to 7% of the base price. [PJC]
For further exploring for "sales tax" in Webster Dictionary Online