If your sending them back to the manufacturer and they will in turn send them back to you after, there should be no duties what's so ever, you are essentially getting it serviced. Also as a Canadian who deals often enough with the USA never send items across the border via FedX always use USA-Canada postal service if you can they will not add extra fees unless they are warranted. Adding to that always send them priority post that was they clear customs faster and with less headaches, also always have insurance for the proper amount. you can also claim in the forms that it is being returned for service.
