I think she was still considering returning it, wondering if it was "too much" - not just that she spent a lot, but that she wasn't sure if it was really right for her.
That was my impression too. She usually does not dress like that but probably bought it to surprise Malik and was second guessing herself or contemplating returning it because later it showed her ripping the tag off as if to say, "I'm going through with it" (both keeping/wearing the dress and sleeping with Malik.