We were in this situation last month when looking for a wedding gift for one of my oldest friends.
She had mentioned she would like vouchers but I felt that was too impersonal a gift to give them vouchers alone given the length of our friendship, so the present hunt began..... initially after searching various shops and the internet I couldn't find anything I was happy with but eventually I found a beautiful handmade vase with a silk rose inside. She loved it and I'm pleased she has something I had hand pick for her to keep instead of just the vouchers alone.
Maybe you could get a small item as well as contributing to the honeymoon? then they will have no idea of the total cost so they want know how to match if they feel obliged to do so.