1. Please excuse my late reply to your very friendly letter of March 1.
2. I hope you will forgive me for not having written you for so long.
3. I hope you will excuse me for not having replied to you until today.
4. I humbly apologize you for my delay in answering to your kind letter of May 5.
5. I have to (must) apologize you for not answering your letter in time.
6. I must ask you to kindly accept our excuses, late as they are.