Offer an unconditional money back guarantee
I've been in this position multiple times. Found it best to offer something that guaranteed my performance or the performance of the product or service I was offering and the assurance that I would fully refund the person or company for any costs incurred or a full refund on the service performed. Put my money where my mouth was.
Offer an ROI guarantee
Similar to above but promising a return on their investment (ROI) at some guaranteed multiple. The ROI could be financial (ex. money saved or money earned). It could be ideas, innovation, convenience or value based (ex. amount of time savings, level of increased or perceived value, ease of use multiplier, etc.).