As others note, you are off on the wrong path. The Montreal Convention limits won't apply to a domestic ticket. So, luckily your claim may be for as much as $3,500.
But, note that the $3,500 is purchase price less depreciation, e.g. fair market value. A shirt which you purchased for $100 and wore 3 times is not worth $100, but likely has a FMV of $10-15. I believe that Amex secondary will apply the same FMV.
As a general rule of thumb, don't ever check valuables in luggage. But, having put them in your carry-on, volunteering to check them may have cost you a lot here.