How can I write an increase price letter to my costumers?

How about:

Dear Valued Customers,

Due to the rising cost of fuel, we must make adjustments to our pricing.

We appreciate your patronage, and regret these increases, however, as a business, we must make adjustments in our pricing to remain able to serve all of you, our most valuable asset, our customers.

Here are the new rates:

It would be helpful to know what kind of business this is concerning in order to give you the best answers here.

Thanks!

And for many, often it is better not to make a big notice of it.
Walmart or Sears or your local plumber does not send notices of higher prices, they merely have their new prices, doing a "MASS" mailing will only give all your customers the idea to start shopping for a new customer, while often merely receiving a new billing that includes an increase while does make some start shopping, it does not make as big of issue as a official letter does.

If you do use some price list, then merely sending a "updated" price list that is obvoiusly higher but not needing to say hey, here is higher prices.

You have made a good point concerning big conglomerates and goods provided, and, perhaps services provided that might be part of a local union for which the customer might not have much of a choice concerning the services that are provided, Fr_Chuck. But, concerning the small guy, such as me who does house painting and piano tuning and repair, among other things for a living, an increase in price can make a huge difference as far as the business is concerned.

I have gained customers because of the simple difference in price of five or ten dollars concerning the services provided. Haven't lost any yet. But, simple price increases and minor differences in comparative prices, can make huge differences in the amount of business that is gained concerning services that are provided. Concerning goods that are provided, that is a different matter.

As such, it would help to know what the business is concerning by massimo.

Do you have another letter