This is not the answer you are looking for, because it's a question - does he know if the recipients value this letter? Often things like this are done year-after-year because they are a habit, and those on the receiving end don't always have a desire or interest in receiving the letter. Obviously I am not saying that applies in your department's case. But in my 22 years of office experience, I've never worked anywhere in which one of these types of letters was sent out, so I am only wondering if there is a need for it.

Instead of telling your customers what great things your department has done, tell your boss to thank the customer for their great order (specify). More personal you can't get it and it tells the customer that you know them personally and appreciate their business.

Who is the recipient of this letter? Does it go to external customers or does it remain internal? Is it sent electronically? It may be too late for this year but if it's an internal electronic distribution, consider a short PowerPoint show instead of a letter. It's brief and with photos and graphics is more interesting than reading a letter.

We have calendars made with our logo and name, address, etc on it. Then we have a group picture taken and mail the picture and calendar to our customers. You should never send a letter talking about yourself. When someone reads a letter they want to see their name or their company's name, not yours. Focus on them and what they have done.