I had to read that three times to get past my interpretation of "The main thing you can do for us is to be here and witness our wedding--but we really want money too."
I would be tempted to take them at their word, write some good wishes on a pretty card, and drop that and only that in the "wishing well."
However, that has more than a little to do with how I value communication: the invitation as sent isn't bad poetry just because of the forced rhyming and such: it's bad poetry because it obfuscates what the writer is trying to say. It's one thing to layer meanings, and quite another to have verse that reads as either "ha, fooled you!" or "I changed my mind but can't be bothered to go back and rewrite."
In fact, I might explicitly wish them a long, happy marriage in which they always were able to communicate clearly with each other. That's something I would wish for any relationship, but saying it explicitly here might be a bit snarky.