Hello,
Problem is that plugin adds the “hidden” class to this field http://prntscr.com/ryfhzq
This class means that block should be hidden and our theme includes appropriate styles. And this is known bug as I can see at the forum of the plugin https://wordpress.org/support/topic/postal-code-missing-under-credit-card-info/
They promised to fix the issue but seems it was not fixed. Contact plugin author again.
Regards