You are able to add text to Invoices with PrestaShop 1.5.  This text actually goes to the bottom of the invoice and is referred to as footer text.  The following tutorial will show how to add footer text to invoices in PrestaShop 1.5.

Adding Footer Text to Invoices in PrestaShop 1.5

1. Login to the Administrator Dashboard as an Administrator
2. Go to the menu bar and hover over ORDERS, then click on the second item that appears in the drop-down menu, INVOICES.  Scroll down past PRINT PDF INVOICE options and you will see:
   
   
   
   
   
   Click on the box to the the left in order to make the FOOTER TEXT field active.
3. Type in the text that you want to appear at the bottom of your invoice.
4. When you are finished entering your FOOTER TEXT, scroll to the bottom of the page and select SAVE in order to save your entries.

Footer text is a great way to share information with your customers using the invoice used for sales of your product.  Here's a sample of the footer text added to the bottom of the invoice: