Our school tried it but i don't think it raised them much money. There were only a few of us who even bothered to sign up and i think i raised half the total amount they ever got with a few purchases through Amazon.
I'm not sure how you would keep on reminding those who did sign up to log into the Giving Machine before buying.
I use TheGivingMachine all the time for my internet shopping and raise approximately £100 per year for my children's school and it doesn't cost me a penny. I became the rep for the High School my boys attend and they had raised about £1000 over 4 years. I sent a message to all the parents, 100 signed up although only about 15 actually use the site and we made over £1000 in my first year - do you now how many cup cakes I would need to make and sell to raise that amount - I didn't lift a finger after my initial mail out.
I just can't imagine why you wouldn't make a free cash donation every time you shop online, even if sometimes it is only a few pence. Like our Grandmothers used to say, look after the pennies and the pounds will take care of themselves. It really is easy to use and takes about 15 seconds to login then get shopping. I was buying a family iPad for Christmas and so was a friend so I purchased them both from John Lewis via TheGivingMachine and I made a £19.00 donation in that one single transaction! It makes you feel good too when you get an e-mail to say your beneficiary has received a payment. Give it a try!