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She's careful about choosing when she's buying clothes.
Literal
She [topic-は] clothing-goods [object-を] buy when [topic-は] carefully chooses.
Two はs in one sentence: the outer は marks 彼女 as the topic, and the inner ときは contrasts 'when buying clothes' against other moments — 'as for the clothes-shopping situation, she's careful' (implying she may be more relaxed in other settings). 衣料品 is the formal/retail noun for clothing — the kind of word you'd see on a department-store sign over the apparel floor, distinct from everyday 服. 注意深く is the adverbial form of 注意深い ('attentive, cautious').