The website, special.usps.com, is not a scam or fake, it is a legit government website. The website was announced a few days ago by the White House, which you can use to order four free at-home COVID-19 rapid antigen tests per household. The website is now up and running.
There may be fake Free COVID-19 Test Kits websites created by scammers that look similar to special.usps.com/testkits. But, the only other official government website for ordering free rapid tests is: www.covidtests.gov.