Yes, that and snails, leeches, copepods, BGA, what else have I received? Not sure about the refund part, but I've received duckweed with fish, shrimp, plants, kind of comes with the turf, I suppose. I love it when a freebie floater is thrown into an order and it's buried in duckweed! Right in the trash, anymore!
It takes forever to remove duckweed from plants, a real pain.
Try removing it, then sending a bill for your time at $100 per hour to the seller!
Duckweed's not the end of the world, but I've had a number of tanks wrecked by snail invasions and hitchhikers you don't want, plants included, aren't welcome by many folks I know.