If you take a package into a PO to ship they'll likely catch an incorrect ZIP code when they process your shipment at the counter. When it comes to a retailer what typically happens is a truck comes up, picks up a whole BUNCH of packages then delivers them to the PO where they're processed automagically. It's far less likely an error will be caught in batch processing. That's why it's very important that the shipper (be it an individual, retailer or whatever) make sure to double check shipping labels or printed addresses before handing it off to the PO. After all, we are talking about the USPS here. (Yes, yes, it's a joke - both of my parents were employees of the USPS when it was still a government agency. My mother retired from there on a medical after many, many years as a mail handler who sorted vast amounts of mail by hand. My dad was a letter carrier and later a carpenter for them. My uncle also retired from the USPS after working for 30+ years as a letter carrier. You might say I have family ties there.)