The article’s solution or answer has been confirmed in practice by a client or Pitney Bowes agent.
Error message: too many message fields on Envelope Designer Plus
UPDATED: 15 June 2015
This message displays if you have tried to place more than eight (8) single-line messages on your layout. Eight (8) is the maximum allowed by Envelope Designer™ Plus.
From a design standpoint you may want to consider whether all those messages clutter your layout and lessen its impact.