Quotes Response Form Do Not Appear In WP-Invoice Plugin

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’.