There are 2 places you can add or edit a message on the invoice/estimate/receipt.
The first way to do this is adding it on the company set up page, which will automatically add it to the bottom of ALL invoices/estimates/receipts.
Click on the user icon in the top right corner of your Dashboard.
On the settings page, click the 'Company Profile' in the top left corner.
Once you are on your company's profile, go down to the Options box and click the 'edit' button on the right side.
A pop up box will appear where you can now add your custom message which can include license info, 'thanks for doing business', etc. Be sure to click the blue save button on the bottom right corner.
The second way to add a custom message on a specific invoice/estimate/receipt from the job details screen, which will only update that specific job.
Click into the Calendar tab.
Next, select the job you want to add a custom message to.
Next you will be brought to the Job Details page of the selected job. Click on the blue '(message to customer)' button to the left of the total price on the bottom of the white box.
Next, a white box will pop up where you can then type your custom message to the specific customer to appear on the invoice. Here you can include license info, 'thanks for doing business', etc.
NOTE: At this time, the invoice/estimate templates do not have a place for you to enter a purchase order number. However, this can be added to the message on invoice/estimate/receipt.