(Instead of asking for cash-Tacky!) Ask family and friends that are close to you, to donate a "needed" item.
Have the brides mother and father purchase the wedding cake. Have the grooms mother and father cover the cost of suit rentals. Have close friends purchase a single centerpiece.
It can be done discreetly, or a list can be made and put on a website.
This way, the couple can cut personal costs from their wedding budget, and someone can buy a gift that they will see in use at the wedding.
This is also great for couples may have a fully furnished household.