Originally Posted by
Often1
AA was correct. There is nothing to "fix" at the airport. Under US law, your allowance for all segments of your ticket, is the allowance for the marketing carrier of the first segment.
That appears to be AA for BOS-LHR. It is up to the first segment marketing carrier to determine which rules it applies and AA does not use MSC. For what it is worth, no US carrier does, so MSC is irrelevant for purposes of any such ticket.
Whereas on a revenue ticket that would be correct, on awards, the marketing and operating carrier are always the same as no codeshares are allowed. Thus, since BA is operating BOS-LHR, their allowance is the one that is applicable for the entire journey. You have a total of 4 legs on your itinerary, so it may not be possible to check your bags all the way but if you need to re-check keep a copy of your trip handy and point out that your trip originated in the US and BA's allowance applies as the first carrier.
I agree with the advice of pay at the airport and dispute later - I once had a similar situation with Turkish Airlines and no matter what I said fell on deaf ears, so I ended up paying the extra charge and disputing it with Chase later - unfortunately for me, they were unaware at the time with the fairly new DOT rule back then, so it took some effort to fix it. I imagine credit card companies will be more familiar with that by now. Good Luck!