Product: Credit card or prepaid card

Due to the global pandemic a flight that I was planning to take was canceled, the Airline offer me a voucher but the conditions of the voucher didn't work for me, I contact the Airline to request a refund and they refuse to do it, however the US department of transportation issue an enforcement notice that passengers should be refunded promptly when their scheduled flights are cancelled or significantly delayed due to any situation ( including the pandemic ). The Airline was not helping to fix the issue at all, even after sending them the notice so i decided to follow the situation with my credit card Company ( Capital One ) As soon as i talked with Capital One they refunded the amount of the flight tickets, i informed them about the documents that i had to proof that i was right to request a refund but Capital One informed me that these documents are not needed until the Merchant reply to them. After two or three months nothing happen until I saw that Capital One Charge me again the amount of the flight tickets so i contact Capital One, they informed me by phone that i need to provide more information, they asked me to write a letter and a few more files that i did not have available at the moment so i start gathering all the information and i finally submit the files on XX/XX/XXXX. I even talked with Capital One that same day because i had an issue with the link so they send me a new one so i can submit the files. 
It turns that Capital One cant longer assist me because i didn't submit the documents on time, the due date to submit the documents was XX/XX/XXXX but i was not aware of the time frame. I was informed until XX/XX/XXXX about the time frame but even then that day Capital One asked me for more time to review the documents that i submitted. 
I called them again on XX/XX/XXXX and they finally informed me that there is no way they can assist me because i miss the due date, during this phone call was the first time they told me about a letter which was the only method they use to inform me about the due date, i told them i didn't see the letter and that i was not aware of the due date until XX/XX/XXXX when i speak with them, I talked with a supervisor that day and she told me that she will check the recorded phone call because she was sure that they must have informed be about the due date by phone as well. 
I called them again today XXXX XX/XX/2020 XXXX to see if they review the records, they didn't and only told me the same story that since i miss the date they cant do anything else. 
I'm not agree with they conclusion, and that is the reason why I'm filling this complain, I'm not agree because they gave me very little time to submit documents that i informed them the first time i talk with them, i realized that i needed to send them the documents after i saw the charge back in my account, not because Capital One contact me to inform me. Also, I was not aware about the due date and the only communication method that they use to inform about the due date was a letter winch I found not a proper method to inform about a time sensitive situation because the letter can easily take the entire time frame in transit. The charge was back in my account on XX/XX/XXXX, I saw it on XX/XX/XXXX and I submit the document on XX/XX/XXXX. If I knew there was a due date I wouldn't wait that long, the problem is that I was informed about the due date until XX/XX/XXXX. 
The current situation is this, since Capital One dispute the charge, the Airline cancel the voucher but is still not responding for my refund, Capital One confirmed that they already paid to the Merchant. I have documents to prove my rights to request a refund but the Airline is not answering my emails and they have told me in the past three days that their system is down and that I need to call them the next day. Capital One not only was unable to assist me but they make the issue even worst for me because the vouchers are cancel and I think that the Airline is hesitating to refund because they are afraid that the bank will dispute the charge again and they think I'm taking advantage f the situation, they didn't told me that but they make me feel it in one of the previous phone call that i had with them. 
I found Capital One 's service incompetent, they set a very tight due date without properly informing the customer when they gave 90 days to the merchant to respond, i pay an annual fee to this credit card so I think I'm not asking to much with a more effective communication from Capital One to the customer about a critical due date.