I'm running Magento 2.2.7. I have both UPS and USPS accounts with the API account information added in and in Production mode.
I have successfully added UPS, retrieve rates on checkout, and can process an order shipment with the checkbox to Create Shipping Label.
However, viewing an order that has USPS as the shipping method, will not show Create Shipping Label checkbox on the Order's Ship screen. I have tried just about everything, and I believe the USPS account is in production mode. I've not been able to locate anyone talking about this as an issue. Is there something I'm missing?