There's a risk it could be counterfeit. You can always hold the item for a week, 10 days, whatever, until you're sure payment is legit. The risk with USPS money orders is pretty low.

There's also a number to call if you think it's a fake: From USPS -
If you think you’ve been given a fake money order, call the Money Order Verification System at 1-866-459-7822
You can also go to the USPS Money Order website for tips on how to spot a fake: https://www.usps.com/shop/money-orders.htm