From NYS DMV:
It is your responsibility to surrender your vehicle plates and remove the registration and inspection stickers from your windshield. Destroy the stickers. Do not give your vehicle plates to another person unless that person will surrender the plates for you at a DMV office. Unless you immediately transfer the registration to another vehicle, surrender your vehicle plates to the NYSDMV.
How to surrender vehicle plates at a DMV office:
You can surrender your NYS vehicle plates at a NYSDMV office. A county motor vehicle office charges a fee of $1 to surrender your vehicle plates at the office. Another person can bring your vehicle plates to the DMV office for you. If another person surrenders your vehicle plates for you, make sure that the person gives you the FS-6T receipt.
How to surrender vehicle plates by mail:
You can send the vehicle plates to the DMV from any location. There is no fee to surrender your vehicle plates by mail to the address below. If you surrender the plates by mail to a county motor vehicle office instead of a state DMV office, you must enclose a $1 processing fee.
When you surrender vehicle plates by mail, the DMV considers the postmark date as the date that you surrendered your vehicle plates. Make sure that you include your current mailing address. The DMV will mail your FS-6T receipt to you. To be notified by the US Postal Service that your vehicle plates were received, send them by registered or certified mail, and request a return receipt. Remove frames and fasteners before you surrender a vehicle plate or vehicle plates. The DMV office will not accept a vehicle plate with a frame or any fasteners attached.
Send your vehicle plates and other information to:
Riverhead Office - NYSDMV
200 Old Country Road - Route 58
Riverhead, NY 11901
If your vehicle plates are lost or stolen:
If your vehicle plates are lost or stolen, ask a police agency in NYS to complete form MV-78B for you. Or, get a report printed on the letterhead of a police agency in the state where the vehicle plates were lost or stolen. Bring form MV-78B or the police report to the DMV office instead of the vehicle plates.
If you cannot get a police report, write a statement that includes your plate number and explains why you cannot surrender your NYS plates. Your statement must be notarized. Bring or send the notarized statement to the DMV office instead of the vehicle plates.
If only one of your two vehicle plates is lost or stolen, surrender the plate that you have. If you surrender by mail, include a note with the vehicle plate that explains that the other plate is lost or stolen. The DMV will process your transaction and note on the registration record that one plate is lost or stolen.
http://www.nysdmv.com/regtransfer.htm#unused
Scroll about half way down to see what I posted above.