This Giving Tuesday people are donating millions to charity. But did you know there’s a way to do it that doesn’t cost you a penny?

For a few years now, Amazon has had a program that sends 0.5% of cash you spend to charity. It’s called Amazon Smile, and once you set it up, (AMZN) gives 50 cents for every $100 you spend to your chosen organization if you use the Smile homepage.

That’s better than a hot stick in the eye, but it might not offer enough benefit to force you to start your shopping from the special webpage, even if you bookmarked it.

But would a solid 5% motivate you?

The clever way to give 10 times more

As Vox noted last year, there’s a loophole that you can exploit to have Amazon funnel $5 of every $100 you spend to charity at no cost to you.

As you probably know, websites can partner with Amazon, linking to their products in exchange for a commission—and (you guessed it) that commission is around 5%. Taking advantage of this very referral tool (it’s available to pretty much anyone), a Canadian charity foundation called Shop for Charity set up an account.

Instead of pocketing the money it receives from its “affiliate links” as they’re called, Shop for Charity donates all of its Amazon checks to deworming efforts through the Schistosomiasis Control Initiative. Deworming is extremely cost effective and make an enormous difference per dollar, and there’s a great need. Hundreds of millions of children are impaired by soil-borne worms, and the treatment is simple and cost-effective, at around $1.25 per child.

To take advantage of this scheme, you can add “?tag=charity48-20” to the end of the URL when you found the item you want to buy, or simply start your shopping at “Amazon.com?tag=charity48-20.” If you save the latter link as your Amazon bookmark, and use it for any and all your Amazon purchases, those dollars will add up quickly.

Ethan Wolff-Mann is a writer at Yahoo Finance focusing on consumerism, tech, and personal finance. Follow him on Twitter @ewolffmann.

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