Final answer:
A balance is the correct tool to measure the mass of a package being mailed.
Step-by-step explanation:
The tool that could be used to measure the mass of a package being mailed is a balance. A balance is a device that compares an object's mass to a known or standard mass. By placing the package on one side of the balance and adjusting the weights on the other side, the mass of the package can be determined.
Learn more about measuring mass of a package