Let me provide some information to shed some light on this feature, customer. 

Please know that the body of the mail in the statement is from the default message. Currently, you can only set one default message, but you can still edit this default message manually when you send your statements.

To edit the body message of the email:

1. Hover to Sales and select Customers.
2. From the Action dropdown, select Create Statement. 
3. Tick the customer to whom you want to send a statement, then hit Save and send. 
4. Expand the Body column, and make your changes. 
5. Once done, click Send. 

Changing the body message won't automatically change the default messages when you send another statement.

If you wish to change or update the default message. Here's how:

1. Go to the Gear icon, click Account and settings. 
2. Click Sales, scroll down, and tap Messages. 
3. From the Sales form dropdown, select Statement.
4. In the Email message column, create your preferred default message. 
5. Hit Save, then Done. 

Moreover, you can download your sales forms and statements in QuickBooks. This way, you can send your customers a physical copy of it. 

You can send me a response if you need additional help or clarification regarding the customer statement. I'm always here to lend a hand.