you would need to create a recordset to lookup the entered postal code, then set a session variable using Recordset Not Empty trigger to signal that the postal code was found. set the value of the session to 1.
in your shipping rule, add a trigger for Based on session variable value:
if the Session Variable "freeShipping" is "!=" a value of "1"
this will cause shipping to be applied only if the postal code is not found.