My guess would be that it's related to their past and that the black haired girl made Iseul choose clothes for her as a way to elevate Iseul's self-esteem and worth in the messed-up relationship they had. And Iseul now realised that she didn't have to do it, the other one flinched because Iseul is drawing more and more lines between them
It's a mistranslation. I've read two different VN TLs of this chapter. The meaning should kinda be: "I can pick the jacket by myself, so leave me alone."
Details:
One VN translation TLed that part as: "I'm not buying this for you. I can choose the jacket by myself!"
The other translation (the one that got posted on this site as well) says: "I can buy this jacket myself? Other customers also shop by themselves, so you can leave."
Tbf, it's a lot of guesswork given the bit of backstory we saw where the mother put a lot of pressure on Iseul (and still does), black-haired chick realised she could use Iseul to gain access to the family, and that the author's other work is WEAKNESS, a toxic GL. So I just guessed that the black-haired girl used whatever means necessary to make Iseul rely and depend on her

Yo I love you so much translator.
Btw I didn't get the part where iseul says "you bought that yourself? Guess you don't need my fashion advice after all" like why did the bitch flinch or why did iseul say that at all, am I dumb?