[treemonkey]

I'm sure you'll find that they pay themselves a decent salary. You shouldn't let it impact yours when you don't even work full time for them.

[londonprinting]

The kudos for helping a charity should be sufficient cost especially if you add your credits or they get some PR from the product/promotion item.

[CarmaCreative]

Or charge a little less than market value, but insist that you can handle their marketing material and web design projects that may be upcoming.