Product: Credit card or prepaid card

I had an offer on my American Express card to be able to take a purchase and use the " Plan It '' offer to space out payments over a period of time for 0 % interest - either over XXXX or XXXX  months at no cost. The offer was expiring on XX/XX/XXXX. I called on XX/XX/XXXX to ask about the offer as my rXXXX needed replacement asap and the cost was just over {$6200.00}. The Amex representative explained the Plan It offer and said if I got the merchant to get the charge done asap I should be able to use the Plan It offer. We had been trying to figure out how we would pay for this large expense and the other option was to accept the offer of a new credit card ( I had an offer for a new XXXX XXXX card that included 0 % intro XXXX until XX/XX/2023 and a {$200.00} one-time cash bonus ). Rather than open a new card, which I could have done, I relied on the AMEX representative. I have been a card member for almost 30 years and thought this was better than opening a new credit card. So I immediately called the merchant and he charged the {$6200.00}. I could see on my online account that the charge had gone through and AMEX listed it as pending. I checked on it again that night and saw that it was still pending so I waited until the next day - the XXXX - to check again. It was still listed as pending. Then at XXXX pm on the XXXX, I got nervous that I was not going to be able to use the Plan It offer ( since it wouldn't let me accept the offer since the charge was still pending, it had to post to the account. I contacted AMEX customer service at that time using the chat feature ( Screenshots attached ). The representative kept apologizing saying he could not use the Plan It feature until the charge had posted. He said something about the merchant needing to accept the funds - I did not know what that meant - all I was told the day before was that the charge had to be made quickly and it should be fine to post in time to get the offer. The representative told me to contact the Plan It representative the next day. I checked again just before midnight and the charge was still listed as pending. Of course the next morning, just hours later after the offer expired, the charge posted. By by that time, the offer from a few hours earlier had expired and I was unable to get that 0 % with no fees. Now of course the offer included a ridiculous amount of fees ( a few XXXX dollars ) including interest. I only used this Amex card because I was told it should be fine. I lost the opportunity to take advantage of opening a different card ( of course that offer expired on the XXXX as well ). I spoke with the PlanIt representative yesterday, hours after the expiration and suddenly the charge had posted but the offer was no longer available. I feel this is unfair and deceptive. It seems strange that the charge posted just a few hours after the offer expired. I would never have used this card for this purchase had I known I would not have this offer. I plan to try to have the merchant return the charge and try to figure out a new 0 % card so I can afford the payments. But I am frustrated by American Express being unfair and deceptive in their offer and not allowing me to accept the offer, even though it was clear I tried to accept the offer within the time allotted and through no fault of my own they did not " post '' the charge until a few hours after the offer expired.