The Order Printer Pro app displays the order number barcode by default and also allows you to show product barcodes on your templates for lookup and packing purposes.
Note: The following guide is relevant if you are using the template designs that are provided within the Order Printer Pro app.
Add the product barcode
The product barcodes can be easily added to your templates.
Steps:
1- Login to Order Printer Pro
2- On the "Manage templates" page, click "Edit template" on the template you would like to update.
3- In the "Code" section, find the following liquid variable (found at the top of the code):
{% assign SETTING_show_product_barcodes = false %} 
4- Change it to:
{% assign SETTING_show_product_barcodes = true %} 4- Save.
Your templates should now have the barcode added to the products that have the barcode field in the Shopify Admin filled.
Add the product barcode text
If you need the barcode text as well, it can also be added by following these steps:
1- Look for this snippet in your template’s code:
<img class="barcode" data-barcode="{{ line_item.variant.barcode }}">
2- Adding this line just below it:
<p class="line-item-sku">{{ line_item.variant.barcode }}</p>
This will automatically show the barcode text below the barcode if you have enabled it to be shown.
The product barcodes and their text are generated based on what you have in the "Barcodes" field on your product variants in the Shopify admin.
If you haven't set them up yet, you can do it directly from your product's 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.
Also, the barcodes and barcode text won't show up until after 4 hours after being 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.
Note: The barcodes are useful for quickly looking up orders or products in the Shopify admin. However, the template does not control what happens when the barcode is scanned since it's essentially just a way to "read/type" the barcode text quickly. Any actions you want to happen when you scan the barcode would depend on where you are scanning the barcode into.