Hank here from Shopify.
Earlier there was an issue with creating shipping labels, with USPS in particular.
This issue has since been resolved in the past hour, can you try to purchase the label once more to see if you are able to do so?
If not, can you attach a screenshot of the error you are receiving, by using the editor similar to the below?
Hopefully, the issue is now resolved, but if you are still experiencing an issue let me know, and I will gladly assist you further!
All the best,