If you order one magnum alongside the regular cases, do this with it: put it at the head table, unopened, with the couple's name and the date in foil on the front and the names of the bride's parents and the groom's parents and the maid of honor and the best man, written by hand in fine-tip silver pen, on the back. At the end of the night, the couple takes it home. They never open it. It sits, instead, on the credenza they buy together in the first house they buy together, ten years from now.
Why this works
The keepsake the wedding party most wants from the reception is something physical that proves the reception happened. The photographs are digital and never get printed. The favors get given away. The guest book stays in a drawer. The signed magnum sits on the shelf for the rest of the couple's life, with the signatures of the people who were closest at the wedding visible on the back. It is the only object from the reception that operates simultaneously as a keepsake, a guest list, a vintage marker, and a credible piece of furniture.
The cost is one additional bottle in a larger format. The yield is the only piece of wedding swag that is still on display at the twenty-fifth anniversary. There is, frankly, no better trade in the entire wedding industry.