I completely agree with Janesmum, I hate how wishing wells pressure people into giving large amounts of money. I give between $50-$100 depending on what I can afford at the time. I refuse to go into debt for a wedding. I don't care if people think I'm stingy, except I'd think its rude of them to judge how much money I can afford to give. I figure our judgments cancel each other out, lol.
Give what you can afford OP. I wouldn't give a card and then anonymous donation though, I'd be clear about what my gift was, it's nothing to be ashamed of.