Who is providing sales and technical support?
JetBrains or JetBrains' resellers and distributors provide sales support to end-users of the third-party plugins.
Plugin vendor provides technical assistance, and tentative SLAs are a part of the Developer Agreement:
You will use commercially reasonable efforts to provide web-based and/or email support to Customers for your Paid Plugins during normal business hours at your location. You will provide to JetBrains a current email address to which JetBrains may direct inquiries from users regarding your Paid Plugins. You are solely responsible for providing all support for your Paid Plugins, and for providing users of your Paid Plugins with all the information necessary for their use of your Paid Plugins. At a minimum, you agree to respond within two business days to any support request that JetBrains identifies as critical, and in all other cases within five business days of request from an end user or JetBrains.