It is amazing that an AC res agent would not know this simple reality of airline fares, as YOWkid notes. This is the worst kept secret on FT and goes back decades. Every backpacker setting out from BC knows its cheaper to bus down to SEA and fly from there, even transiting through YVR. Be careful about booking some cheap fares to ensure they do earn AE miles. Cheap SQ and TG fares may earn a percentage or no miles. Make sure codeshares are indeed booked with AC flight numbers too.