[NAME, COMPANY AND ADDRESS, ex.
John Smith
1234 First Street
Suite 567
Anycity, Anystate  85245]
 

Dear [NAME, ex. John Smith], 

Enclosed is my rent check for [MONTH, ex. September].

I do understand this payment is late and I hope you will accept my apology.  You have my assurance that this will not occur again. 

Sincerely,

[YOUR NAME, ex. Jill Jones]