A few years ago, Nick Cave was invited by Gap to appear in one of their ad campaigns. He sent a terse letter in reply.
Dear Gap.

I might put on a pair of your jeans if you to pay me £1 billion, but even then I would have serious reservations.

Signed Nick Cave.
In a recent interview regarding his new album, Cave explained the letter:
I get letters from people telling me they got married to The Ship Song, or that they buried their best friend to Into My Arms, and I don't want them to look at the TV and see that they buried their friend to a Cornetto ad or something. I feel some sense of responsibility about that, even though they wave enormous sums of money at you. That's where my muse puts her foot down.