Can I compose a headed letter on Quickbooks?

Yes, you can, glyn-owen92. 

On the Account and Settings page, you can modify the Message you'll want your Customers and Suppliers to see. 

Customer

1. From the Gear icon, choose Account and Settings. 
2. Click Sales from the left menu and then click the pencil icon under the Messages section. 
3. Change the settings according to your preference. 
4. Hit Save once done. 

Supplier

1. Still on the Account and Settings page, click Expenses from the menu. 
2. Click the pencil icon along the Messages section. 
3. Choose your preference and type your message afterwards. 
4. Then, Save and Done. 

You can also check out this article if you want to customise your sales forms. 

I'll be here if you need more help with your email templates. Have a good one!