Elias here with Shopify Support. Thank you for reaching out!
I understand you're looking to change the address to your P.O. Box on your shipping labels.
First, you'll need to add your P.O. Box address to the Locations settings and assign the inventory to your locations. If you have multiple locations, you can also set the priority of locations for fulfilling orders. Once that's done, you can change the shipping origin in your Shipping settings and the shipping labels will pull the address from your shipping origin.
Let me know if this is what you're looking for?
Should you have any further questions, don't hesitate to let me know!