How can I edit the contents of the printable invoice for Ubercart order?
Try Ubercart Register Invoice, this will allow you to define new email templates for invoice emails ... these same templates should also be visible/useable by your sites Theme.


  Registers invoice templates with Ubercart so they appear on and can be
  selected for the Ubercart "E-mail an order invoice" Rules action.
  
  Registers invoice templates with Drupal so they can be used to theme
  the invoice data.
  
  Provides extensive instructions and feedback on how to configure
  invoice templates.


See the README. You name your template: uc-order--*.tpl.php and it will be seen by the system. Also see the built-in help (prior and after creating a template) at http://mysite.com/admin/help#uc_register_invoice