On September 17, 2018, 
FHAULK). After booking on August 4, 2018, the American website redirected to a page showing the following baggage policy, which I luckily took a screenshot of:
Note, the line that:
Based on your travel, one airline is designated as the Most Significant Carrier, and that airline's baggage allowances and charges apply to your entire journey.
The initial itinerary contained three flights: MMH to LAX via AS 2189, LAX to RDU via AA 782, and RDU to PHL via AA 2081. Due to Hurricane Florence, our itinerary was updated, keeping the same Alaska Airlines flight 2189 from MMH to LAX, but with a revised second flight of AA 506 from LAX to PHL.
Here's the flight information for the two segments we ended up flying:
Given this itinerary, I was sure that American Airlines (and not Alaska) would be classified as the Most Significant Carrier. I confirmed this with the American representative when updating our itinerary due to Hurricane Florence. Hence, I was under the impression that the American baggage policy would apply where I have a free checked bag as an Aviator Red cardholder.
However, the Alaskan agent at check-in charged a $25 bag fee. While I don't usually check bags,
Initial message
After returning to Philadelphia, I filed a complaint via the Contact American webpage on September 26, 2018. The text of my complaint was:
Greetings, I am an Aviator Red Cardholder, which entitles me to a free checked bag. When booking my award travel from MMH to PHL, the checked baggage policy indicated that the policy of the Most Significant Carrier would apply. The first leg of the flight was an Alaskan Air flight (AS 2189). The second leg was an American flight (AA 506). I read into the rules determining Most Significant Carriers and American seems like it should be the most significant carrier, as the flight was 6 hours versus 1 and crossed 3 time zones versus 0 for the Alaskan flight. I had contacted American prior to my trip to confirm that American's baggage policy would apply as the Most Significant Carrier. However, when I arrived at MMH, the Alaskan agents required a baggage fee of $25 for my checked bag.
Can you please reimburse me or credit me in some way for this expense? I have a screenshot of the Most Significant Carrier page from booking and the receipt from Alaskan for $25. I don't see anywhere to attach them.
Autoreply
I got an autoreply with the title "American Airlines Has Received Your Request", containing a Reference Number of 1-27822290351.
American Reply
On September 27, 2018 I received the following reply from Neya Marie from Customer Relations, with reference code AA Ref#1-27822290351.
Thank you for contacting American Airlines Customer Relations.
We received your request for a refund of the baggage fees you incurred. While we've elected to exempt some customers from paying the baggage charge when traveling on American Airlines, we cannot compel other carriers to do the same. Accordingly, we must respectfully decline to make an adjustment. We are sorry to disappoint you but hope you will understand that we are not in a position to refund fees collected by other airlines.
Dr. Himmelstein, our position does not lessen your importance to us. We value your 20 years of loyalty in our AAdvantage® program and are eager to continue the relationship we have enjoyed over the years.
My Reply
Thanks for your quick reply and explanation. You are correct that Alaskan Airlines (and not American) collected the baggage fee at MMH. However, when I booked this award travel with American, the website indicated there would not be a baggage charge. See the following screenshot I took when booking:
Note the following quote: "Based on your travel, one airline is designated as the Most Significant Carrier, and that airline's baggage allowances and charges apply to your entire journey."
I looked into the rules determining Most Significant Carrier (MSC) and it seemed that American would be the MSC for this itinerary. Furthermore, I called American to confirm. Hence, I was wrongly given the impression that my American baggage allowance would apply to this itinerary.
Here is the copy of the baggage receipt for $25 from Alaskan Airlines.
While I understand you cannot directly refund this charge (as it was with a different carrier), perhaps you could credit my AAdvantage account a commensurate amount of miles. Furthermore, I think it would make sense to review the baggage policy page during checkout which claims the policy of the MSC will apply, when in my situation that appears to not have been the case.
American Reply
All of the following messages are between me and Neya Marie on September 27. The reference number for this response is AA Ref#1-27823162003.
Thank you for contacting American Airlines Customer Relations.
I apologize for the misunderstanding regarding the benefits that come with your AAdvantage® Aviator Red MasterCard. With your card, your first eligible checked bag is free of charge for you and up to 4 traveling companions on your reservation, when traveling on domestic itineraries operated by American Airlines. For more information regarding the benefits that come with your card, please visit AA.com, under the AAdvantage credit cards tab.
While we acknowledge your continued request for reimbursement of your baggage charge, we must confirm that our position remains unchanged, and we must respectfully decline your request for a credit or reimbursement as it was charged correctly.
Dr. Himmelstein, thank you for allowing us to address this matter. We hope this issue won't jeopardize our business relationship. We would consider it a privilege to have your continued patronage.
My reply
Thanks again Neya for your quick reply.
Can you please address that my award reservation receipt included the following note: "Based on your travel, one airline is designated as the Most Significant Carrier, and that airline's baggage allowances and charges apply to your entire journey." I called before our travel to confirm that American's baggage policy would apply as the Most Significant Carrier for this itinerary.
My concern is that baggage policy was incorrectly advertised on my reservation. Had it been properly communicated, I would not have brought a checked bag to the airport and would instead have used Priority Mail by the United States Postal Service.
Reply from American
The reference number for this response is AA Ref#1-27823313192.
Thank you for contacting American Airlines Customer Relations.
We have received your additional comments and appreciate the opportunity to respond.
The screenshot of the page you send to us states that one airline is designated as the most significant carrier and, although it doesn't state which airline is the most significant carrier, it would be American Airlines, as they were the first marketing carrier. However, because Alaska Airlines is the one that charged you the fee, you would need to contact them for any refund that is owed, as we did not collect the funds.
Dr. Himmelstein, we appreciate your business and look forward to welcoming you on board a future American Airlines flight soon.
My reply
Thanks you Neya Marie for the response. I'm glad we agree that American was the Most Significant Carrier, and that therefore the American booking confirmation (as screenshoted) indicated the American baggage policy would apply.
When I got to MMH airport, the Alaskan agent and I had a similar discussion. She said that Alaskan has no baggage agreement with American Airlines (since 2018), and that despite what the American website had indicated about the MSC policy taking effect, Alaskan would have to charge me to check my bag. Since I had arrived to the airport early, I called American and waited on the phone for ~30 minutes. The American agent said there was no way she could get Alaskan to waive the fee and suggested I pay the fee and contact AAdvantage customer service following the flight. This is what I am doing now.
I do not think Alaskan will refund me because according to Alaskan, they followed their policy. The issue here is that the American website indicated the fee incorrectly, by writing that the policy of the MSC applied.
In two weeks, I will fly to Montreal with American, earning Gold status. It is disappointing to encounter such resistance from American regarding rectifying an issue caused by a misadvertised baggage policy, especially in light of my continued loyalty. Can you please escalate this ticket to a manager who can more effectively address the issue and make sure it doesn't happen again?
Final Reply from American
The reference number for this response is AA Ref#1-27823950618.
I received your reply and see that you're still unhappy with us. One of our primary responsibilities in Customer Relations is to help our customers who have experienced a situation such as yours, which is why your correspondence was forwarded to me.
It was clear to us when you brought your concerns to American's Corporate Headquarters that you felt strongly about this issue and were escalating it beyond our other customer service teams. In effect, our previous response serves as the supervisor/corporate involvement you request. We are provided guidelines by our management and executive teams and Customer Relations is the highest level of escalation there is.
While all cases are only handled at this particular level, we do consult with upper management, especially when our original responses/resolutions are not accepted. I have reviewed and revisited all the particulars concerning this matter. I recognize that you may disagree with our decision, however I'm not seeing any additional information in your most recent correspondence that would change our handling of this matter. We are very sorry to have to deny your request for reimbursement or compensation at this time.
Dr. Himmelstein, we appreciate your business and look forward to welcoming you on board a future American Airlines flight soon.
Conclusion
So in conclusion, the American representative admits that the website statement that the policy of the MSC applies is misleading, but refuses to rectify the issue or bring my complaint to a supervisor. Surely at this point, more than $25 has been wasted by refusing to admit the mistake with the webpage.
Hopefully, this post will get the attention of someone at American who can help rectify the problem and fix the confirmation webpage with the misleading baggage information. Furthermore, I'd love any advice from savvy travelers on the situation or how to avoid this in the future.