I think that might be an extras chapter, like in the first volume. 5 chapters in volume 3 matches the second volume's number of regular chapters, too. The manga was basically done as an advertisement for the LN, as far as I understand it, but still, this ending sucks.
That's certainly an ending. Thanks for finishing this LN ad. I found a Yen Press copy of this manga, and it seems like the 7.5 chapter uploaded here is supposed to be the extra chapter in Vol. 3.
The actual 7.5 are three 4komas + bonus text story which includes a reference as to why Routa doesn't want Mary to bathe him, as shown in Chapter 8 .