Student or Learner
This is not the first time I have purchased clothes from your outlet but this is the first time I have experienced a problem. Of course, I do understand that problems of this kind can happen from time to time. I, too, should have been a bit more careful while choosing the item. Right now, what I am hoping is that you will take measures to replace this faulty shirt with a good one. I can show you the receipt if it is necessary.
Expecting a prompt response,
Would it be better if the sentence in bold if I changed the words in bold to "I expect a prompt response"?
There's no need for such a rude demand when complaining for the first time.
Leave it out altogether, or say something more polite, like
'Hoping for a favourable reply'.