I ship live insects via USPS Priority. I add a sticker to the outside of the shipping box that indicates "Hold For Pickup" so USPS knows to keep it at the destination post office. This process works great.
What I need to do is integrate the customer's phone number on the shipping label with static text, like "PLEASE CALL 702-555-5555 to notify customer" - USPS will call that number upon arrival at the destination post office.
I don't want to do this with every shipment, not every customer wants their phone number on the label. I would like to add something at checkout that says "include phone number on label", and then have that appear on the shipping label when selected.
I am currently not using any custom apps or integration, I am open to shipping integration that does this, or if it is available within Shopify somehow, etc.