To run Scan and Ship you need barcodes on your:
a) Products
b) Invoices
Product barcodes can be set up in the shopify admin on the Products > Variants page.
More details here
The barcode labels app can be used to print barcode labels for you products .
If you run the shopify order printer you can add barcodes to your invoice label template by using a liquid tag ( see instructions here )
Most other applications allow you to add a order number barcode to your packing slip, email info@scanandship.com telling us how you print your packing slips and we will let you know how to add a barcode .
You can find out about the equipment needed to run Scan and Ship here