First off you will need to set the app to use either exact postal code matching or partial postal code matching.  

Within the app click Settings

Scroll down to the Delivery section and locate the Validation block

Select Exact postal code match or Partial postal code match

Click Save

How to specify the valid postal codes

Now you must tell the app which postal codes are eligible for delivery. You do this by modifying your Outlets delivery settings. 

Within the app click Outlets

Click on one of your outlets' names to reveal the edit page

Locate the field called Eligible postal codes

Enter a comma separated list of your valid postal codes into this field and click Save

Task complete! You customers will now be required to enter one of your eligible zipcodes before proceeding to the checkout process. 

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