So do you think that it could be the acorus? Sounds like it. No aquatic plants I know with thin leaves like that stay firm out of water. You had better go get your money back. I'd be ticked too. Maybe someone else will have a suggestion, but I really think that is what it is.
It is too thin to be dwarf sag, and it is not any form of hairgrass or U. gramnifolia, and def not blyxa, so I will opt of non aquatic.~ Go get your dough back~