Issue We have a weird thing going on that is not consistent.  Even though we have the PC entered in the customer card, some of the PC's print and some don't.  I don't have a clue as to why this is happening.
Solution Requested updated data from client, but wasn't able to re-create the problem.  Postal codes are currently printing on invoices for these customers.

Discussed this with client and determined that these invoices were probably created before customer cards were updated with postal codes.

Advised that country code could be deleted, it has only been filled in on a few customer cards currently, and isn't needed for invoicing, could conceivably cause a problem on future invoices.
In addition, sometimes country codes are set up with or without postal codes. If a country code has no postal setup, then it will not print on invoices, chqs etc. Even if the postal code is entered on the customer card.

To check country code setup:

General Ledger > Setup > General > Countries
Choose country and look at Address Format column
Make sure correct address format is entered.

Example: City + State + ZIP Code