"have had issues with being able to ship letter mail to the US from Canada. "...
and it is against Postal regulations to do so...
Sold Goods/Merchandise do NOT qualify as "lettermail" as per Postal guideliens/policies/rules and regulations.
Goods/Merchandise must be accompanied by customs information>item description/value/country of origin,etc.
Cheapest option by Canada Post is Tracked Packet.
If shipping via other carriers, research their options.