Scan and Ship allows you to scan a barcode to check that the envelope you are using is the correct one for the order. To do this you need to add a barcode to your address label.
If you run the shopify order printer app you can add barcodes to your address label template by using the following liquid tag:
<img src="https://app.scanandship.com/barcodes/{{order_number}}" />
If you cannot add a barcode to your address labels you can skip this step by unchecking the "Check Address" setting :
You can access the settings by clicking on the cog icon in the app.