Jonathan Lau

CubePM is a private messaging system for WordPress.
Unlike other PM system, CubePM allows messages to be sent to multiple users and also displays the whole PM system on the frontend using a short-code.

CubePM

CubePM is a private messaging system that integrates right into the frontend of your site.

Shamim Hasan

Front End PM extension to integrate with WooCommerce. User can contact seller directly from product page. Option to set so that user can contact seller only after purchase.
<a href="https://wordpress.org/plugins/front-end-pm/" rel="ugc">Front End PM</a> 
<a href="https://wordpress.org/plugins/woocommerce/" rel="ugc">WooCommerce</a>
This two plugins need to be activated first before activating this extension.
Once activated, user can send message to seller directly from product page. 
You can go to Dashboard > Front End PM > Settings > Misc > WooCommerce to set if you want your users can contact seller only after purchase. You can also set in which order statuses customer can contact seller.
<a href="https://www.shamimsplugins.com/docs/front-end-pm/integration/front-end-pm-woocommerce-integration/?utm_campaign=wordpress&utm_source=readme&utm_medium=description" rel="nofollow ugc">More Details</a>
Github
<a href="https://github.com/shamim2883/front-end-pm-woocommerce-integration" rel="nofollow ugc">https://github.com/shamim2883/front-end-pm-woocommerce-integration</a>

Front End PM – WooCommerce Integration

This is very helpful plugin to integrate with Front End PM and WooCommerce. Users can ask pre-sale question or seller can provide support after purchase within wordpress.

Nice plugin to integrate with Front End PM and WooCommerce

User can contact seller directly from product page. Option to set so that user can contact seller only after purchase.

CubePM vs Front End PM – WooCommerce Integration