$ are charged are supposed to always be based on the costs of the operating carrier. It usually doesn't matter if the second flight is being operated by someone else. Assuming routing on the way back was the same, OP wouldn't be charged if AC doesn't charge for international and using them first.

On the other hand, I have seen airlines be cogniscint of the full itinerary before and not charge as a result. GF went YYZ-JFK-MAA this summer, with the first segment on DL, and then interlined to Jet Airways. She was not charged, apparently because DL looked at the second leg, even though not operated or marketed by DL, and considered it an international itinerary.