I think he's buying it for himself; what she's providing as her thanks is her time and expertise. She's pretending like it's an imposition on her to do this for him, although we know better.
The text leaves it ambiguous, at least in this translation, but that's the most reasonable interpretation I can see.
I don't get why everyone in this universe mistakes Igarashi for a child. Even when the sugoi kawaii desu art style is considered, she still dresses and lives like an adult.
Wonder why the salesman assume igarashi as a child.... i mean size wise you could assume that, but would any child wear that cloth? Re reading it again i find it weird....