(NEXSTAR) – The United States Postal Service might have a special delivery for you today. The agency started delivering free at-home COVID-19 test kits in mailboxes and at doorsteps Monday.

People who ordered their allotment just after the website relaunched last week received tracking information Monday morning alerting them the delivery was on the way. Each package contains four tests plus an information card on the test kits’ expiration dates.

That’s because the tests being sent out may be past their originally designated best-by date.

A package received by a Nexstar employee contained tests by Celltrion DiaTrust with the expiration date Feb. 2, 2023, printed on the packaging. But the Food and Drug Administration says these tests aren’t actually expired.

The agency has extended expiration dates for at least 19 different brands of COVID tests.

In the case of Celltrion DiaTrust, the expiration dates have been pushed out a year, making these tests accurate until at least February 2024.

You can see if any COVID tests you have are approved for an extended shelf life on the FDA’s website.

It’s important to pay attention to tests’ expiration dates because they may break down over time, which risks providing inaccurate results.

This is the fifth round of free at-home tests delivered by USPS. The Postal Service has delivered 755 million tests in previous rounds.

Households that haven’t placed an order still have time to claim this latest batch of four free tests. You can do so at COVIDtests.gov.

It’s not clear how long the window to order new tests will be open, though the Department of Health and Human Services told the Associated Press it would be up for several months.