Offer an unconditional money-back guarantee
I've been in this position multiple times. I 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. the amount of time savings, level of increased or perceived usefulness, ease of use multiplier, etc.).