Step 3: upload the folder with the plugin via FTP client to your WordPress site to the plugins folder, then activate. Or upload the archive with the plugin through 'Add new plugin' WP admin interface, then activate it.
Using the plugin
Upon the plugin activation it adds two new settings on the product edit page.
"Send inquiry" settings
The first setting enables the feature. The second setting (optional) changes the label of the button that opens a modal window with a form for sending inquiry request.
The modal and the form are look like this:
Upon submit of the form the administrator of the site receives an email with product options chosen and custom message/contacts of the customer who has submitted the form.