Have you received a postcard like this in the mail?
If so, notify the USPS Inspector General. You can file an online complaint here: https://www.uspsoig.gov/form/file-online-complaint
After receiving these postcards and noticing there was no return address, I became suspicious and looked up the phone number on the card online only to discover that this is indeed a scam. There are several news articles that date back as far as December 2019 alerting Sacramento residents about this: https://sacramento.cbslocal.com/2019/12/17/parcel-hold-scam-sacramento-shipping/
Always remember that if it seems suspicious, it probably is, and don’t forget to notify the appropriate authorities.