You can not use Fedex boxes through USPS. The USPS has their own boxes for customers to use or we sell others at each office if you don't have any.Can you find a FedEx package without a tracking number?
You can track the FedEx package as well without having the tracking number. So, in case of lost FedEx tracking number, all you need to do is: Switch on your desktop or laptop and go to the official site of the FedEx (www.fedex.com). Select your location if it asks for that.