After you print a Passport label, you will need to affix the label to the outside of your parcel, then hand off your package(s) to USPS at the nearest post office or mailbox, or you can also have the parcel retrieved from your door.
You can arrange a pick-up using the USPS Pickup Scheduling tool: https://tools.usps.com/schedule-pickup-steps.htm.
The label should be applied flat with the barcode and tracking number clearly displayed to ensure seamless delivery to your customer(s). You can just reprint the label at no extra cost if it becomes damaged and can't be scanned.
As a reminder, you only pay for the parcels you ship - not for each label printed.