I forgot to mention that I put up a small PostCheck update a couple days ago.
In certain situations, the USPS will return multiple possible correct addresses for the address you’re trying to standardize. This is most common when your address is missing a specific unit number. In some cases, each unit has its own ZIP+4 Code. The USPS returns a default ZIP+4 Code to PostCheck which is not always the correct one for your address.
The 1.8.2 update no longer adds the +4 portion to the ZIP Code when multiple addresses are found, but will still fill in the standard 5-digit code. This is the safer way to go and should make batch-updates more reliable.