I don’t get it. You’re saying you don’t know how to communicate the value of what you do. This is where the real sell begins. It’s called handling concerns. And you had the immense good fortune that they already said yes.
Get them to remind themselves why they hired you in the first place. What would happen in their business if they didn’t have the result they were hiring you for? Put a dollar value on that. Suppose what you did would be worth half a million dollars to them. For a measly $15k fee. They’d be crazy not to do it.
If you don’t know how to sell the value you bring, you’re in the wrong business.
If they don’t value what you bring, you shouldn’t have been talking with them in the first place.