The issue has been identified and a fix is being implemented.
Posted 3 months ago. Apr 03, 2019 - 10:55 EDT
Some stores are temporarily unable to purchase USPS shipping labels from the fulfillment page in the Shopify admin. Shipping options are working normally for customers at checkout--only the purchase and printing of USPS shipping labels from within the admin is affected. We’re investigating and will keep you updated.