The story
From mail + piece; part of the business jargon of postal services and mail marketers for more than a century, but not common among the general public until the 21st century.
Etymology from Wiktionary, CC BY-SA 4.0.
mailpiece
Today it means: (countable) A piece of mail, that is, an item of (physical) mail; especially, an item of flat envelope form factor as contrasted with an item of package form factor. Full definition →