Yes, so that you don't need to go to My eBay for every buyer, you can add a message that will automatically be applied when every order is marked as shipped. 

  • Go to 'Settings - Configuration - Store Settings'. 
  • Scroll down to 'Your Orders'.
  • In the field labelled 'Default buyer feedback comment' add the message you would like to communicate. For example: Good buyer, prompt payment, valued customer, highly recommended. 

NB: There is a maximum of 80 characters.