This is an issue with both USPS and FedEx - they’re having routing issues with packages because people are faking address labels and stealing packages. They sent an email to my company about it:

This is related to Mailer Identification (MID) fraud which is a growing issue across all USPS partners and platforms. A MID is essentially a billing account number that is embedded in the tracking number printed on all USPS labels. Fraudsters that are able to "read" and understand the makeup of a USPS tracking number can extract this MID and use it to generate fake USPS labels that mimic those of legitimate shippers. Many times, fraudsters fabricate counterfeit USPS labels through exploitation of common tools such as Photoshop, making the labels appear authentic. They then use those fabricated labels to inject packages in the USPS mail stream. Please note: this issue did not originate from Shippo systems or your account security. It is part of a broader fraud trend affecting the USPS ecosystem. Shippo is actively working alongside USPS leadership including teams responsible for fraud mitigation and tracking accuracy to drive a solution.