I'm not certain USPS international money orders work with Canada and they're expensive, and limited to $700.
Western Union has locations in Canada for in-person transfers. They also offer direct/wire transfers and on their app.
There's international prepaid cards.
Gift cards? Maybe if you're alright with spending it all at Costco or Starbucks. There's probably quite a few gift cards that'll work in the US and Canada.
I guess a lot of it depends on how much you can trust a buyer and the protections/security that are available for each method.
I remember the days of C.O.D and when I could send cash without much concern. That was when public pay phones and personal pagers were still used.
The story of three wheels and a man...