The best evidence to support the statement that in the late 1800s a law made it easier for magazine companies to do business is:
"This was a boom time for magazine publishing, owing part to the developments in offset printing technology as well as to the Postal Act of 1879, which had granted magazines discounted mailing rates."
This statement directly references the Postal Act of 1879, highlighting how it facilitated business for magazine companies by providing them with discounted mailing rates.