Hi Rainstorm--Long time no "see." I have the same issue with my duck eggs, although I'm having pretty decent luck this season because they are hanging out outside and only going in the duck house to lay. They used to not lay in the house at all, but putting a few golf balls in there has encouraged them to use the nest nearly every time. They still manage to get the eggs muddy by digging down to the dirt and then covering the eggs (and golf ball, lol) in straw, but at least they're not also covered in mud.
But if yours are making messes in the nest boxes, then I don't really see how to fix that. Just understand that people who have hatched ducks before should expect that the eggs will be dirty. I would simply wipe off anything that comes off easily (do it right away before it dries on), and ship them as-is. Let the purchaser know ahead of time so they don't freak out. Just so you know, my hatch rates went up several percentage rates when I stopped washing the eggs. I hardly represent a scientifically significant sampling, given that I've only hatched about five batches of eggs, but I thought I'd mention it anyway.
Good luck, anyway!
