I want to send an apology message to my customer.

You could say that. Mr. xx, it would be better if you knew his last name. For example, if it is Mr. Li, you can send it like this. Dear Mr. Li, hello. Who should I call today? Write clearly. I'm sorry to bother you, because you are very busy at work. I wanted to meet you, but I didn't want to offend you. If you give me a chance, can I apologize in person? If he comes back, it doesn't matter. Don't answer, just apologize when you have time.