When I try to reply to tweets via Support, the allotted character limit is always less than Twitter's standard character limit. Why can't I use all 280 characters in my Twitter replies?
Since tweets are limited to 280 characters, when replying to another user's tweet, the character limit of their user handle is subtracted from the 280 characters allowed. The remaining character count can be used in your replies.
See Setting up your Twitter channel if experiencing trouble setting up the channel.