T.H.
Please shop off their registry. If you absolutely don't want to, at least include a gift receipt. I would think about $50 would be acceptable but if that's too much then just spend what you can. I also suggest shopping on line (through their registry) and having it mailed directly to them before the wedding. I remember when we got married that it was so much easier to deal with the presents that came to me beforehand so I could put them away and get thank you notes written easier.