Hopefully you have a way to contact the customers. The zip code is required for manually entering a card. https://squareup.com/help/us/en/article/5176-manually-key-in-card-payments-without-the-square-reader
Manually entering cards and doing offline transactions are risky because there's nothing Square can really do if you don't have accurate information for a valid credit card when it comes time to actually process the transaction. Be sure to have contact information for any customer that isn't charged at the time of the sale.
