It's been a VERY long time but I ordered boxes directly from USPS and they shipped them directly to me in Toronto.
One thing I recall is that I had to go through a USPS Business Facility, the regular supplies people couldn't do the International shipping part.
Can you order them and have them shipped to ChitChats in Buffalo and they can bring them over the border for you? I haven't used ChitChats recently, if they don't offer that service you could try Cross Border Pickups, they are reasonable, $17 for up to 30lb, $20 for up to 100lb.
CBP also do Canada to US services just like ChitChats or Stallion, I have never used them for that purpose so I have no idea how efficient they are in comparison, presume the pricing is basically the same.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
- answering Trolls is voluntary, my policy is not to participate.