Do you have a tracking number? And does it say specifically that the item SHIPS from Canada? The last time I paid $12 shipping and thought I'd have parcel tracking from a seller within Canada, it turned out they were drop-shippers for a UK firm and my money did, in fact, go towards airmail from overseas. But, like you, I bought local because I needed it fast so there are several factors to consider here.
Without tracking, your seller can't prove delivery so it's short-sighted of them to charge $20 to ship something that a buyer would reasonably expect to receive via Expedited Parcel at least. Managing buyer expectations is important.
Personally, I would not pay $20 CAD for domestic service by lettermail but what did the item listing actually say under the Shipping details section for the $20 in shipping that you paid?
p.s. You can't get insurance on regular lettermail, nor mail anything of significant weight with it. That's the reason it's lettermail. Even third-party insurers would **bleep** an eyebrow at that.
