Hello there, @mhorncpa.
I'll guide you to where to find the letter templates so you can use them to describe the QuickBooks services you will provide for your clients.
For the time being, letter templates aren't directly listed under the Accountant Tools in QuickBooks Online Accountant (QBOA). But I've got a direct link to help you access those templates outside of QuickBooks so you can download them: Engagement for QuickBooks Services (Letter Templates).
Also, you may want to check out this article as your reference to guide you and your team in performing clients' tasks this year-end in QBOA: QuickBooks Accountant year-end checklist.
Please feel free to leave a comment below if you have other concerns about managing clients in QBOA. I'm always ready to help. Take care, @mhorncpa.