Here is my essay draft of formal letter to complaint one thing.
And I complainted bad service of one online shop.
I want to get some suggestions for my revising.

To Whom It May Concern:

I want to complaint about my last shopping experience of your online shop. One month ago, I ordered two skirts of the same style but in two colors, black and white from your online shop.
First, I have been waiting for over almost three months for the same things without reasonable explanation. Secondly, I received wrong product twice after I asked customer service about to returning the products. Third, the servicer told me that the Black Cats delivery person would take the products from my dorm and she at a schedule I find convenient but the delivery person didn’t show up as scheduled. So I sent the products back by myself via the post office instead. During the waiting time no one contacted me and I paid for the postage of returning wrong products twice without any compensations or discount.
This was a really inefficient process and unpleasant experience. I wouldn’t shop on your online shop again unless you are willing to improve. I thought the high evaluation on the online shop could be trusted but I was really disappointed. I hope you would have much better staff training, storage controlling, close connection with cooperating delivery company and make your delivering process. I like your products but it’s a pity to have such poor service. I am looking forward to hearing from you.