Currently, QuickBooks Online (QBO) doesn't have the option to send letters to customers all at once. And while this option is not yet available, I encourage you to visit our QuickBooks Blog so you'll become updated the newly improved features of the product.
However, if it's an email that you're trying to send to all of your customers, you can do it in QBO through Outlook. Just make sure you have installed an Outlook and set as the default email service on your computer.
Below are the instructions on how to achieve this:
Click Sales on the left navigation panel.
Select Customers at the top.
Place a check-mark next to the customers that you need to email.
Click the Batch actions drop-down and choose Email.