I guess I need to set up some Fast Payment buttons to see where the problem lies… can you tell me which guide you read for your implementation? Was it this one?
While I get to configuring that, you could try what @emre suggested: use an Ask Question
action to display the Change, instead of a Show Message
action. Like this:
Action Name: AC Show Change Message and Logout Confirmation
Action Type: Ask Question
Question: [:message]
Buttons: Ok=y
Rule Name: RU Ticket Closed Show Change Message
Event: Ticket Closed
Action Constraint: [=TN('{CHANGE TOTAL}')]>0
message: Change: {CHANGE TOTAL}