To what extent is a Runner forced to take actions to pay for a Tollbooth?
From the official FAQ, regarding Tollbooth:


  The Runner must pay 3 [credits] if he or she is able to do so, even by spending temporary credits (such as bad publicity credits).


Also from the FAQ, on encountering ice:


  When the Runner encounters a piece of ice, he or she must resolve any abilities triggered by the encounter before he or she can trigger paid abilities or break subroutines.


If there are at least 3 credits on Ghost Runner and/or Net Mercur when Tollbooth is encountered, then they must be spent upon encounter. If the Runner doesn't have 3 credits available to spend at that moment, without triggering other abilities such as Cloak's, then the run ends.