How do I add a suffix to the price field? I'm using drupal commerce in my current project, I want to display the price like "$299 excluding tax/month", but the price field is not editable by default.
there is a way to add this suffix to the price?
Do you need to include the value in the field? If not, you could just put it in place with a template file. You can create a template file that formats that specific field, for example; see Drupal 7 Template (Theme Hook) Suggestions.

Otherwise, you can use the Simple Field Formatter module.


  The Simple Field Formatter module allows site administrators to easily control the display of field values with following features.
  
  Features:
  
  
  Prepend the field value with specified text/HTML
  Append the field value with specified text/HTML
  Reverse the Field Value
  Trim the Field Value
  Convert the field value to lowercase letters
  Convert the field value to uppercase letters
  Convert the first character of the field value to uppercase
  Convert the first character of each word in the field value to uppercase (7.x-1.x-dev)
  Replace some characters with some other characters in the field value (7.x-1.x-dev)
  Link the field value to node(7.x-2.x-dev)