I received a letter from an art magazine and they offer me a small amount of money to let them publish some of my artwork.
I think is unnecessary, for many reasons, and i'like to write that i agree with the publication, but i refuse the money.
I'd write something like this: "i appreciate your offer, you can publish my work, but there's no need for money, really. I'm just glad that you like my work".
Maybe "I appreciate your offer but I am happy for you to publish my work for free / without payment" I can't imagine anyone being offended by this. You could also request that they give the money to a charity of your choice.