I don't live there now, but I grew up in the Deep South; specifically, the more French part of Louisiana. AFAIK, Bride's Boxes are pretty standard throughout the Deep South and south Texas, though they may have a different name for them in different areas.
It isn't a true freebie, of course, in the respect that the couple or their parents have paid for the food and drink, but normally it is not charged as two extra plates. It is just portioned out as part of what was paid for, but it acknowledges that the bridal couple very seldom actually get to eat at their own party because they are too busy running around greeting all of their guests. Making up a generous picnic box for them is just a really nice way to thank them for their business and make sure that they do get to share in all of the good food that they paid for.
Let's see ... it's been 27 years, but as I recall, ours included champagne, slices of both wedding cakes, a pint each of crab mornay and spinach madeleine, a half-dozen or so each of stuffed mushrooms, sliced sausage, glazed meatballs, cold shrimp, and beef ravioli; a few chicken wings, a small packet of cold sliced ham, roast beef and cheese, raw veggies, a couple of bread rolls, some condiment packets and lots of butter crackers. It was a very substantial picnic for two.