It is possible to use the Custom Question to trigger Auto Actions,
Trigger tab :-
Booking Field Does Not Contain = Custom Question X - (leave next field blank) then this This auto action will only trigger if the Custom Question field is filled.
Message tab :-
Enter the text you need to send to the guest with/without a payment link if there is additional payment required.
== Option 2 - If you want to view/keep the original charges made on the 'original booking' ==