I agree, $50 is more than enough considering all you've already done! Do you definitely want to give money? Did they register somewhere? If so, maybe you could find something nice but inexpensive on their registry or even just get something for their home like a set of placemats & napkins, a cookbook & pasta bowl, or salad set, or even a hurricane vase w/ a candle in the middle and have it sent to the couple so you don't have to bring it to the wedding.
Good luck with whatever you decide!