Dear Mr. Mohsin,
Do you wish to join them, or do you only want to politely decline by saying "No, thanks"? If you are not joining them, a simple thank you letter will suffice.
Steps to Respond to an Interview Invitation
On the other hand, if you wish to join after being selected in the interview, there is no need to send a lengthy letter. Simply acknowledge the receipt of the invitation and express your eagerness to attend the interview. Also, mention that you would be delighted to join them once you are selected.
All the best,
Kumar.S.