Simply replace the unfilled areas such as "__" and "_[date]_" with your information.
We regret to inform you that [I/we] have decided to close down [my/our] business. Our last day of business will be on [date]. After that date [name of company] will no longer be offering [type of products/services]. The decision to close down this business was not an easy one and [I/we] enjoyed the relationships with [my/our] customers over the years.
[I/we] would like to thank all of [my/our] customers that [I/we] have done business with over the years. [Name of company] could not have been as successful as it was without your loyalty.