So make it opt-in. TBH though, calling a labelled opt-out option "trickery" is a little low.
Can't tell you how many times my habit of 'just click continue' has gotten me onto mailing lists I had /zero/ inclination to be on simply because that box comes already checked when downloading something.
Even happens when I'm LOOKING for it, but I get distracted by a phone call or something else. Not saying I'd fuss about a dollar, but I once got these /awful/ chocolate dessert things from dominos. At first I thought they were a free promo, since I didn't order them. Nope. Turns out they were in a suggestion screen during checkout. Now I look, but if I had a different pizza place to order delivery from, that experience, and their ever-present attempts to get my extra money for things I do not want, would have me getting my pizza elsewhere. As it is I only order it now when I absolutely /crave/ pizza.
So yeah, I am on the side of trickery. If you didn't specifically add it to your cart, adding anything else, charity or otherwise, is kind of sneaky and underhanded.
Make it opt in or don't do it.