When I bought my 870 a few years ago they were running a rebate program. A coupon came with it in the box and it didn't specifically say U.S. only, or not valid in Canada, so I sent it in. I think it took a couple of months, but a rebate checque (US$50) did come.
Regardless of what it says on the web site now, if I bought a new Remington and found a coupon like that, I'd risk a stamp and send it in. You might get lucky.