Honestly OP I’m very curious about this ‘everyehere’ that you’ve looked for decent clothes. What is the difficulty, exactly? 14+ is a completely average size. And no one has to look for a separate category of clothes because they are over 50.
I’m hoping that by ‘mail order’ you mean ‘online’? (Rather than an old style paper catalogue dropping through your door?) So you have unlimited choice.
Can you say what sort of prices you’re happy to pay? If you are not a completely regular size and shape then very cheap clothes will never be very flattering - but I know people who swear by Sainsbury’s t shirts ...
But Toast would supply you with all you need. You may want to avoid some of the more garish patterns. And the dresses are sometimes odd. But they can’t be bettered for good, simple t shirts, shirts, trousers.
If you have a lot more money then Margaret Howell could be perfect.
These can both be bought online if you can’t get to one of their shops.
(By the way, my experience of trips to the US is that it’s better to take very little and shop there to wear there.)