Possible Clues:

• "Get back to us" letters
• 'Get back to us' letters
• Say 'Sorry, can't make it,' e.g
• ___ card, part of a wedding invitation
• "Tell me if you're coming to the party," briefly
• Accept an invite, perhaps
• ''Get back to us''
• It might mention a plus-one
• Republican's second in command, please reply
• "Are you coming?" request