Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear [name],
There has been a change in our delivery-service policy that we would like to bring to your attention.
We have always provided free delivery for any orders placed with the [firm], regardless of the size of the order. Due to the increase in the price of fuel, we must now limit this free delivery service to any orders over $[amount], not including [restrictions]. We regret the necessity of imposing this rule.
Thank you for being one of our most valued customers.
Sincerely,