I think she is baffled with Rinnosuke's take on it. He actually read the insrctions and didn't understand, so the product is exactly what it is. Just, if people think that's transparent cloth, what will people think of those who buy such things?
Well, to be fair, Rinnosuke's ability to see a function apparently only goes so far. He thought the use of a Game Boy was to destroy people and worlds, until Yukari pointed out that his conclusion was true insofar as it applied to virtual people and worlds (I think this was a story in Curiosities of Lotus Asia?). So, maybe in a similar vein, he knows that what Sanae is holding wraps things, but only concludes that it's meant for wrapping bodies.
Well, she picked it up knowing perfectly well what Saran Wrap is and how to use it. His calling it the technically correct name of "transparent cloth" surprised her because she probably never had considered using it that way till now.