You will need coupon code 50OFF for that price. Example of dates: Saturday February 2 to Friday February 8th. You will be flying via Fort Lauderdale (middle of the night to and from Plattsburgh). If you only want to go as far as Fort Lauderdale, Spirit will let you do it for $268, including the bag.
Coupon expires October 22 at 23:59.
PS. Parking at Plattsburgh airport is no longer free. From the Plattsburgh bus station, a cab will run you $15 each way. Greyhound.com is the company that does the Montreal-Plattsburgh route (and onwards to New York City). By bus, please allow an hour buffer for border delays above what is scheduled as Greyhound actually waits for stragglers. Plattsburgh airport has free wifi.