If you spend £20, the 0.5% donated equates to 1p. Amazon can afford to do better than this. If they abandoned their main site and donated from every purchase, this would be a much more worthwhile gesture. Also, their main charities are huge charities who receive plenty of support already and don't have great track records which leaves nonprofits with tiny support systems wanting. This is essentially a marketing scam by a vile company which refuses to treat its workers with any semblance of respect. They want you to spend their money on their site and they are doing this by making you think you are doing a good thing, when the help you are doing is essentially negligible for the charities that really deserve your help.
I'm not saying this site can't help a little bit, but it is a total marketing scam covering up for years of disgusting treatment of workers and tax avoidance.
Really, you shouldn't be shopping at Amazon at all if you truly want to do a good thing for the world.