Agreed. As it is a utility and simplistic product, it will not have many more features added later. Hence I sold lifetime deals with promise of all future upgrades and support.
Then ask yourself a question - how big of a support you gonna provide ? I would limit support hours to an annual amount you can accept, and treat larger needs beyond that limit as a separate development work order. Prepare a well-done documentation with examples, so you can say - for your own peace of mind - that support covers things not described in the documentation.
Due to nature of the product, most of the support is only needed during onboarding. Its pretty straightforward afterwards with full documentation available.
I guess I am not being able to generate enough trust in prospects even after 14 days no-question-ask-refund policy.