Hinata actually had a good idea. Miyako can't handle most people, but if the cashier is her sister then she'd be okay and she's be able to get out of the house a bit more. Every little bit counts.
Finally, a realistic representation of what it's like to get a cold! Back as a kid whenever I got sick it never bothered me much, but I'd use any excuse to get outta school. But it seems like a cold is the equivalent to living death for most manga characters.