Picture of your kids in a Christmas card: Abbr

• Bus.-ltr. abbr
• Bus. letter insert
• Env. insertion
• Something inside: Abbr
• Env. stuffer
• Stamped return env., for example
• Pkg. insert
• Abbr. with a check
• Britannica, e.g.: Abbr

Wiktionary Word definitions in Wiktionary
abbr. enclosure (qualifier indicating that a letter is accompanied by further material English)