$2 towels versus $20 ones. The one who tricked herself by not checking out the vending machines is you, Kogasa.

Just like Sima Yi, you overlooked the obvious and thus feel into Zhuge Liang's gambit.

(and yes, Zhuge Liang/Kongming was tagged in Pixiv.)

Reader-added tags include "Kongming's trap", "Good at business, in a way", and "This must be the work of the gap youkai".