This is Messenger Plus!' sponsoring. Think of it as the tab bar in Messenger itself. The buttons can come and go based on which advertisers are interested.
You can disable those extra buttons completely:
- Click on the Plus! logo in the contacts list.
- Go to 'Preferences & Options' -> 'Customize!' -> 'General'.
- Untick 'Show me promotions associated with Messenger Plus!'.
- Click 'OK' and restart Messenger.