French to English Marketing Advertising / Public Relations Advertising

This (and several other terms that will be coming up) is a term from a glossary I'm translating for the advertising department of a TV station. I understand the concept, but can't put my finger on the right term in English. Here is the description given:

"Remise figurant aux conditions générales de vente des supports publicitaires (...) accordée en fonction du volume publicitaire et/ou de la part de marché et/ou de la progression réalisés par le mandataire sur l'ensemble de ses mandats dans un support, groupe de supports ou régie publicitaire..."

Elsewhere, I've translated "mandataire" as broker or agent, if that helps or hinders matters...

Thanks for any help you can offer.