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.