I guess I don't understand what you expected of Amex. Their purchase/return protection policy is very clear. The suit is no longer in its original condition and even if it was, they cap their return protection at $300 per item. The item was not lost, stolen or accidentally damaged so purchase protection does not apply. What is Amex supposed to do? Amex has made a lot of goodwill gestures to me since I became a member, but it wouldn't even occur to me to try to get them to cover what was clearly my own mistake.

Were I the OP, I would do a few things. First, look at the labels on both the jacket and pants to make sure they are the model and size you ordered and make sure the pants, jacket and receipt labels all match. Brooks Brothers has 4 different cuts of suit, each progressively slimmer than the last. A 40R in the Milano cut is going to be far slimmer/tighter than a 40R in the Regent cut. Assuming all the labels match up, I would physically measure the jacket's length (both arm and neck to bottom down the back). Mislabeled suits are rare, but it can happen. Sleeve and body length are the best way to figure this out. If it is drastically off or if any of these labels or receipt don't match, you *MIGHT* have a case to make at Brooks Brothers.

MIGHT being the operative word here, as they are under absolutely no obligation to accept a return, exchange or refund of an altered item. Suits are always sold as a set, no business anywhere is going to take back or exchange half of a suit set. In all likelihood, the model's cut has changed in the years since the last purchase (very common these days as suits get slimmer and slimmer) OR the manufacturing process is just inconsistent. No matter the reason, the responsibility is ultimately on the customer to ensure proper fit BEFORE alterations if they refuse to do a fitting at the store.

In all honesty, I have no idea how someone could buy a suit AND have it altered without trying on either piece. I wouldn't remove the label of a plain white t-shirt without trying it on first, never mind a thousand dollar suit. Mass produced clothing items will always have inconsistencies in the manufacturing process. How did the pants even get altered without them being on the person's body? How were the buttons going to be placed on the suit without checking the sleeve length first? The pants being tighter than normal would have been a huge tip-off that something was wrong with the sizing as all BB suits are a Drop 6 from chest to pant waist. I've had a lot of pants altered in my life and not once has it been done without wearing the pants for the tailor, usually with the shoes I planned to wear with those pants. Even if you have a similar pair of pants to compare it to, every pair of pants is going to sit a little differently on a person's body depending on the cut so the pant length may differ slightly from pants to pants. That's especially true if the customer's body changed in any way. Any tailor that doesn't have you try pants on before tailoring them is either your personal tailor who knows your body as well as your spouse or a fraud.

Unfortunately, this is going to end up being a very expensive mistake. You can always try pleading your case with Brooks Brothers, especially if any of the labels or measurements from my second paragraph above are off, but anything they do for you is going to be a goodwill gesture. They have no obligation to do anything here, nor should they. Amex certainly has no obligations at all.

If pleading your case to Brooks Brothers fails, I highly recommend trying to sell it on eBay to recoup some cost. Altered and never worn or worn once suits are a big business on eBay, I personally spend a lot of time looking for deals on suits just like you're describing on eBay because I'm a weird in between size (Between 38 and 40R and Drop 8). I bet you could get $500+ for it easily, especially if the pants still have most of the fabric left to let back out a few inches. Worst case scenario, you can have the pants let out and keep them while donating the jacket so you at least get some use out of it.