David Welzman says

Let me make this Clear

If I don't deliver to you- you don't owe me a cent
That's a promise no one in the marketing industry makes

I will come to your trade show exhibit, create crowds of people around
your stand, I'll deliver your main messages, I'll introduce
your sales team and compel most of the crowd to take action,
such as leaving their contact details, or enquiring about
your products/services.

and if for whatever reason, your displeased with my first performance
give me a hug and send me on my way, that first performance
will have been my gift to you

That's how confident I am that I can deliver on my promise