Apology plus reship
When to use: Use this when the customer got the wrong item and you are sending the right one immediately.
Subject
Sorry -- the right item for order order number is on its way
Hi customer name,
I am sorry -- we sent you the wrong item on order order number. That is our mistake, and I want to get the right one to you as fast as possible.
Good news: your correct item is already being shipped and should reach you within timeframe. You do not have to wait for anything on your end for that to happen.
I will follow up separately about the item you received by error so returning it is painless. For now, the priority is getting you what you actually ordered. Sorry again for the mix-up.
Best,
your name, company name