Amazon has been giving shoppers the ability to choose a charity for donations via their "Amazon Smile" program for the last couple of years. The program is available for both the US and UK versions of the site, though the US version has more options and gives you a lot more information about how much your purchases have contributed.
The idea is really simple. You buy what you're going to buy via Amazon (UK or US) and the company donates a percentage of your eligible purchases to your charity of choice. There's no extra cost to you, so setting it up on your account is a bit of a no-brainer.
As I'm based in Ireland I don't buy via the US website that often, though I do use it to send gifts to friends in the US or when I'm in the US for a couple of weeks. (Clothes and various other things are quite a bit cheaper!)
I do, however, use the UK site quite a bit.
If you're buying from Amazon you might as well do so via Amazon Smile and help a charity of your choosing.