Why not charge for support?
And if it turns out to be your mistake (faulty product or missing documentation) as opposed to something the user could have reasonably solved by themselves, refund the charge and possibly provide compensation for the inconvenience.
Because if you charge for support but refund it if it's the company's fault, the company now has a big financial incentive to never admit it's their fault.
Companies used to charge for support.
But if one company stops doing it, eventually everyone has to stop doing it.
Then the race to the bottom begins...