Help! Shipped item and then received notification from Mercari that “Your transaction was cancelled due to an issue on your buyer's end.”

Literally 30 minutes AFTER I shipped the items the person purchased, I received this notification and now the buyer is nowhere to be found. Mercari notified me that I can reach the post office, but the post office said it’s already in transit and there’s nothing they can do. Am I just out my money and the product? TYIA for your help!