You have a piece of 3rd class mail, which is printed matter, advertisements and such, which could be mailed at 1 cent per 2 ounces, even to Canada. Per 3rd class regulations, the envelope was unsealed. Notice the side flap which could be pulled open for inspection, which must have been done and customs assessed for some reason. It would be interesting to see the contents.
The flag cancel was used at Pleasant Hill, Ohio 1914-1936, and is uncommon, rated at 40 points in Langford's Flag Cancel Encyclopedia. The absence of the month/day in the dial is consistent with a 3rd class use.
The Canadian customs duty stamp was applied by the sender ...
Jarnick, How would foreign (non-Canadian) mailers know about the customs regulation and bought stamps for application?