Lachia never said they were the suppliers of the ice crystals -- they just endorsed Levory's store in their catalog...which sells their ice crystals...but people don't know that. And like regular magazine catalogs, the magazine's company include their own products and other products (usually from companies that pay to be featured or are endorsed by the magazine's company).
But, in life, when people [like Lachia] endorse an item, they usually get perks out of it OR are the item's supplier [like Lachia being supplier of the ice crystals]. So the nobility is assuming Lachia is the supplier and spreading the rumor. But like 'zoe' said Ran wanted the rumors to go around (probably to be the known as the ice crystal trendsetter) and now she has to tell the Emperor that the ice crystals are from Lachia because it was gonna come up sooner or later and they probably wanted to convince the Emperor to not raise their taxes because of it (and that scene was skipped over for some reason - as 'zoe' said)