I write to ask you to please send your replacement item that we ordered as soon as possible and to ensure that such errors do not happen again, or we will have to look for that instead.
I am writing to ask you to please send us a replacement product as soon as possible and to ensure that such errors do not happen again or we will have to look elsewhere.
I am writing to ask you to kindly send replacement goods we ordered as soon as possible and to ensure that such errors do not happen again, or we will have to look elsewhere instead.