Got my first sale, wondering if I am being scammed

Made my very first sale on 12/5, shipped it same day via USPS, and it was delivered to the customer 12/7. They filed for a return today, stating “Item arrived too late”. I run a small 3D print farm that I’m trying to get off the ground.

Is there any chance this is some sort of weird scam? I don’t understand how two day shipping isn’t fast enough.