The tone of the letter should be that you are reluctant to raise prices but that you still believe you are competitive. Stress your strong points and be explicit about what services/products are affected. If some key items are not affected, it might be worth highlighting those as well.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
Unfortunately, nothing stays the same forever, and effective <date of price increase> we must raise the price on <products or services affected>.
The new prices will be:
- <list of products/services, quantities and new prices>
We regret that we have to pass on this increase, but it will allow us to continue to provide high value <products/services> to our customers. This means that we will continue to offer high quality with competitive prices.
We appreciate your business and look forward to serving you in the future.
encl: <List of enclosed items goes here>