Barcodes are a great way to speed up your order lookups and packing of products.
You can include barcodes in your templates by doing the following:
Login to Order Printer Templates (Shopify admin -> Apps)
Click the "Customize template" button on your design
In the editor, go into the "Options and formats" section
Scroll down to the "Barcodes" section, and enable the checkmark for order barcodes, product barcodes, and/or the product barcode text:
These product barcodes will be generated based on what you have in the "Barcodes" field on your Variants in the Shopify admin.
If you haven't set them up yet, you can do it directly from your products information, as illustrated in the screenshot below:
Once added, new orders will show the barcode for the updated products. Shopify will not update old orders when you make product changes, so only new orders will show the new barcode image or text you have set up.
Note: The barcodes won't show up until after 4 hours after configured. The app imports all barcodes on a 4 hour cycle, so they'll show up automatically as soon as the app has detected them.