how to write email to customer for the given issue?

Dear Mr A.

Our store policy, which is clearly posted in the store, is to allow return of any clothing item within 3 days. Since you did not return this item within the allotted time, I'm afraid we cannot allow a return.

Just keep it simple.

However, if Mr A is a regular and good customer, I would consider two things. First, the condition of the dress. Is it in unworn condition? Second, the resalability of the dress. Do you believe you can resell it, maybe at a discount?

If its unworn, you might say to Mr A that since he is such a good customer, you will make an exception this time. Maybe charge a 15% restocking fee. If its sellable, you might take it back under the condition that you can resell it and tell Mr A you will give him whatever you can sell it for.

First if A is a regular customer and buys high quality items, why is the store not considering an exception. If I was a long time regular customer known to the store, it is not out of the norm to expect special treatment.

Next if you are refusing, it is very impersonal to send an email. I would as a customer consider that in poor taste.
Why not a phone call to talk to them and perhaps make some offer to try to make them feel better about it