You can create cover letters to cost estimates. Cover letters are created in the Settings tab
Open the settings by clicking on the cogwheel icon on the bottom of the blue sidebar.
Click on Cover letter templates
Start creating a new cover letter template by clicking on New template
Name the cover letter template
Give it a headline (i.e. order confirmation, terms of contract, request for quotation etc.)
Write down the text you wish to add
You can leave the other headline spaces empty
You can edit the existing cover letter templates by clicking on the ellipsis symbol on the left hand side of the list. Click "Open"
Remember to save all changes once you're done!