My suggestion is to actually call the store and ask, you could even record it if legal and upload and write in your letter to amex recording of return attempt is available.
The suggestion of recording made me curious so I checked and here are the U.S. states where you can record a call without the other party's explicit consent, as long as you are an active participant in the call:
Alabama, Alaska, Arizona, Arkansas
Colorado, Connecticut, District of Columbia, Georgia
Hawaii, Idaho, Indiana, Iowa, Kansas, Kentucky
Louisiana, Maine, Michigan, Minnesota, Mississippi, Missouri
Nebraska, New Jersey, New Mexico, New York, North Carolina
North Dakota, Ohio, Oklahoma, Oregon, Rhode Island
South Carolina, South Dakota, Tennessee, Texas, Utah
Vermont, Virginia, West Virginia, Wisconsin, Wyoming
The states where both/all parties must give their consent in order for recording to be legal are California, Delaware, Florida, Illinois, Maryland, Massachusetts, Montana, Nevada, New Hampshire, Pennsylvania and Washington.
As long as both you and the other party are in one of the one-party consent states, then you can record the call without the other person's consent. Strange thing is, though, that I've often recorded calls to businesses whose automated system informs you that they're going to record the calls for customer service training purposes or whatever, and one time, I made a call to complain about a problem with a hotel I was staying at. The recording clearly stated that the call would be recorded, so I recorded it, too. The customer service agent ended up getting rude with me, so I told her to be careful what she says because I was recording the call, and she told me that she would end the call if I didn't stop recording. Not sure what the legality is of the second party recording a call if the first party has already stated that the call was being recorded. Just another example of how businesses and the government like to do things that they don't like us doing back to them. But I digress...
A variation of your suggestion for people who want/need to make the return attempt in-store would be to use their cell phone to record the discussion, whether a video recording or audio recording. On the one hand, it seems pretty pathetic that we're even considering having to do things like this just for a Return Protection claim when we never had to do anything like this before, but on the other hand, I guess if it helps expedite our claims and protects us from any accusations that we've lied about our return attempts, maybe it's not such a bad idea?