Wants to submit I-751 documents to USCIS vermont service center. To mail documents what is best option UPS, FedEx or USPS?

The USCIS Service Centers are organized around P.O. Boxes to sort their incoming petitions. Our preference is to use USPS, and use Priority Mail, to get delivery confirmation. You can use UPS or Fedex, but they must deliver to a street address. The application then needs to be sent to the mail room and sorted. That is two extra steps, where things can go wrong. By the way, you have the street address for VSC wrong. The P.O. Box 200 is not part of the street address.

You can use the postal service to forward (to the address you cited ) your submission to USCIS by certified return receipt requested or priority mail with receipt confirmation. You will need the street address to forward by courier.

USCIS certified with a return receipt is the best option to that address and to provide the protections you wish.