I have flown YUL - AMM three times in the past few months, all in J. At check-in I have noticed an option to "buy-up" to J from Y every time. Does anybody have any experience with buying up at check-in? What typically is the cost? Is the buy-up option frequently available JFK-AMM AMM-JFK and ORD-AMM AMM-ORD or only DTW / YUL?
Thanks!