Thanks so much for your helpful tips!!!  I have included the postal code after the province in the "region" tab as suggested.  

I have sucessfully e-filed two ways:

1.  I have successfully e-filed a return with the postal code 23700 (dropped letters in postal code and added two zeros as suggested by the ProConnect customer representative.

2.  I also have successfully e-filed a return with the postal code 00000 for another client per your suggestion.

So, it looks like both are "valid" options going forward...my preference would be to put 00000.

Thanks again!