IMO the far and away best option is to take it to a UPS Store. Verizon has a contract with UPS to return equipment, just walk in with the device(s), UPS will take it from there - box the device(s) up, get your information, ship it free, and give you a receipt which you absolutely should keep and use the tracking number to track the shipment to make sure it gets to Verizon. As I have upgraded my equipment over the 10 years I have had FiOS I have returned devices 3 or 4 times, everything has gone perfectly each time.
FiOS TV: Extreme HD, Internet: 50/50, Digital Voice
VMS Enhanced Service: 1 server, 2 clients
Keller, TX 76248 (VHO 1)