Support is included with every active license — answered by the people who wrote the code, not an outsourced ticket queue.
Check the documentation — most setup and configuration questions are answered there, and every plugin ships with a built-in Help tab in its own settings screen.
Use the contact form and include your license key, the plugin and version you are running, and what you expected to happen versus what did. That is usually everything we need to help on the first reply.
Every purchase carries a 14-day money-back guarantee — see the refund policy. If a plugin does not fit your store, you get your money back without a debate.