In most of cases issue is in that your Page Template that you use for Invoices/Quotes does not support comments.
Read this to understand how to add comments support for page template:http://codex.wordpress.org/Function_Reference/comment_form
The other frequent question is ‘How to display the list of existing responses?’.
So, to get responses on quote page use this standard wordpress function:
http://codex.wordpress.org/Function_Reference/wp_list_comments
Actually, these function usually used for regular wordpress post types. You can find code examples in default themes ‘Twenty Ten’ and ‘Twenty Eleven’.
