I am hereby submitting the manuscript entitled "......" to major consideration for publication in your journal "....". I declare that the manuscript has been prepared based on guideline of your journal, never been published and solely submitted to "...."(NAME OF JOURNAL). I am looking forward to hearing your decision in the earliest time.
Often a Letter is a short scientific article without the usual framework of introduction-MM-results-discussion. It reflects on an earlier publication in the given journal, either supporting or challenging it's findings.
You will receive such invitations once you have published a scientific article, the journals are simply trying to attract more authors. Consider whether the invite comes from a respected journal you wish to publish in & if you already have some data available to write this Letter around.