They took an extra £80 from my donation, as a kind of 'tip' to themselves!
Now I know, I should have been more careful to check everything at every stage, but I did not expect this. I had put in my donation amount, and then I clicked on agreeing to giftaid it, and at this point they just added it on!
I've emailed to ask for a refund, but the emails I get back don't fill me with hope. Lots of 'Hope things are now resolved. If not, please do this extra things' - I think they are waiting for me to give up.
Looking on trustpilot I'm not the only one this has happened to!
Please be v careful to double check at every stage if you make a donation via justgiving - and if you are setting up a donation page then please use some other platform!