The United States Postal Service aka US post office does send legitimate text messages. But not on WhatsApp.
There’s been scams going on that claim to be from USPS but are not. It usually contains a link and a time limit to make a decision, making it clear that such messages are nothing but scam. The US post office has addressed such message details in their post on their website as well.
If you receive such message from any number, please ignore them. And most importantly, you MUST NOT click the link or take it seriously.
Here’s a video report on USPS messages:
