As of the moment, there isn't an option to create and customize a letter in QuickBooks Online.
But don't worry, I'll pass your request to our product developers. This way, they'll know of your need to create a letter in our software.
In addition, you can also visit our QuickBooks Online Blog so you'll be updated with all the new features released in our software.
Lastly, you can always visit our Help Articles page for QuickBooks Online if in case you need to learn some easy steps for any of your future tasks.
If there's anything else that I can help you with, please let me know in the comment section down below. I'll be always around ready to help.