According to the United States Postal Service website:
Mr. ZIP was based on an original design by Harold Wilcox, son of a letter carrier and a member of the Cunningham and Walsh advertising agency, for use by a New York bank in a bank-by-mail campaign. Wilcox's design was a child-like sketch of a postman delivering a letter. The figure was used only a few times, then filed away. Later, AT&T acquired the design and made it available to the Post Office Department at no cost.