Let me make this easy.
My guarantee to you is that every time we work together, you will learn something and you will get closer to achieving your goals. Otherwise, I’ll refund your money.
Of course I cannot guarantee you will achieve your goals because I can’t guarantee what you do when we’re not working together and I can’t guarantee that your goals are achievable or that you have the necessary skills, patience or determination to keep going.
But I do guarantee that if you turn up, pay attention and take action, you will succeed.
For what it’s worth, I’ve never had to honour this guarantee.