Payment by Results is a service in which will send out your adverts to teachers, and then you pay on a “commission only” approach based on the amount of money you have received or invoiced, with the percentage of the commission agreed in advance. 

First off, you need to email This email address is being protected from spambots. You need JavaScript enabled to view it. with details of the product or service on sale, how much you charge for it, and (if you have it) a link to a web page describing the product or a copy of an email that you have sent out advertising the product.

There are more details here:  Payment by Results advertising