Yes, if you are shipping to the US via Canada Post (and most other carriers) the tariffs must be paid in advance.
FYI - If you are wanting to add the tariff costs to what the buyer pays you should add it to the shipping charge rather than the item price. If it's added to the item you will incur more eBay fees, if you add it to the shipping charge you will not as eBay fees are based on the lowest of what you charge the US buyer or what you charge for a domestic sale. In most cases the domestic shipping will be cheaper and thus fees will be lower.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
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