On September 1, USPS changed their “First-Class Mail Parcel” Service to “First-Class Package Service – Retail.” Magento 1.x and 2.x merchants offering this service must change the name, as “First-Class Mail Parcel” will no longer work. A Magento 1.x patch is available in MyAccount and on the Magento Open Source Download Page (see SUPEE-10336). Magento 2 updates will be available later this week.
In the meantime, you can work around this issue by updating your USPS shipping method (see Technical Bulletin for instructions). Please note that you must revert these changes before deploying the Magento patch when it becomes available.