그들은 주문한 상품을 1주일 내에 받지 못한 것에 대해 많이 불평했습니다. But here's a thing. 불평하다 is normally used as an unofficial complain that you are whinning to your friends or yourself. To someone you are close with. So it's just showing your feelings that you don't like it. But if your complaint is delivered to the seller verbally or in written, then 불평하다 is not likely to be used. But we may say 컴플레인 (=complain)하다. 컴플레인하다, as a loanword, is used in casual conversations for the complaints submitted/delivered to the seller/organizations.