Before writing the letter about the price increase, call the organization and ask if they change details that do not change the head (so as to write a letter you will be on his name). If you send a document with the incorrect data, it may not accept, it could worsen relations between organizations.
Never start a letter with news of the promotion. This may entail termination of the contract. Start your letter with thanks for your cooperation. Next, you can describe the main advantages of your organization (how many years of work, achievements). In separate graph, describe all the positive qualities of your product and working with you. If your product has some changes for the better, you should note it.
Next, write about the price increase. Emphasize that you are also comfortable with this. Describe all the nuances, which changes the cost, for example, the increase in prices for raw materials, utilities, costs of customs clearance, etc., Indicate all positions affected by the increase.
If the prices for your goods has not changed for a long time, be sure to specify exactly how much. It will positively affect the perception of your writing.
Be sure to specify the date on which the effective date of the new rates. Never write a letter like that, in hindsight, warn customers at least a couple of weeks.
If clients have any discounts, specify that they continue to operate. You can write a specific price without discount and with discount.
The writing style should be business, don't apologise several times, it can look ugly.
At the end of the letter make sure to sign up (showing your position), mark the date and don't forget the phrase that you hope to continue cooperation. It is desirable that the letter was not very long, enough to write a ½ page A4.