Sorry to disagree but the postal rule cannot apply to responses to invitations to treat, because any response to such an invitation could not, by definition, be an acceptance.
Whether any particular advert is an invitation to treat or an offer is a different question. If you decide it is an offer then maybe the postal rule could apply. (Although it is quite hard to think of good examples as in most cases the things that lead you to conclude that in some particular case an advert is an offer would usually also lead you to conclude that the postal rule must have been impliedly excluded).