@gerlandog, here is an example of using constraints on Actions within a Rule, taken from Gift Certificate Tutorial. The Rule is called GC Redeemed (used as Payment) - Update GC Data.
In particular, the last 2 actions in this rule GC Redeemed (used as Payment) - Update GC Data have a constraint to determine whether to refresh the payment screen or close the ticket… only one of the actions will execute because of the constraints on the 2 actions:
Action: Display Payment Screen
Constraint: [:RemainingAmount]>0
Action: Close Ticket
Constraint: [:RemainingAmount]<=0