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(is this correct?)

Peace and all goods!

Dear Sir/Madam:

Thank you for the invitation for Sales Asistant Interview.
And i apologize for late reply bacause now is the only time i check my mailbox.

I am kindly inform you that i will not be attending the said interview due to some reason.
Hoping for your kind and understanding.

Yours sincerely
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Thank you for your invitation to the Sales Assistant interview. I apologize for my late reply, as I have only recently checked my mailbox. Unfortunately,I must inform you that I will not be able to attend this interview due to an outside conflict I cannot control. I appreciate your kind understanding.

Welcome to the Forums. I'm confident you'll find this a good place to come to when you have questions about English.
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Hello rain, and welcome to English Forums.

What is the "some reason"? Have you found another job already? Would you like to schedule the interivew for another times? How long was your delay in responding?

If the person sent you an invitation to interview, his or her name should be on there - use that person's name in the e-mail.

Dear Ms. Anderson,

Thank you for inviting me to interview for the position of sales associate. I apologize for the delay in responding, but I was unable to check my e-mail for a few days. Unfortunately, I must decline your invitation to interview, because I have just accepted a job elsewhere. I am grateful for your consideration.