Not sure what terms are your trying to negotiate upon. But if your wish to do this formally you will have to do it by way of a formal contact. Not other way. Again, because if limited abailable information from your side I don't know what terms are you trying to negotiate but have a look at the following sample sales commission agreement. It'll give you an idea of how formal agreements look and have one drafted for yourself.Sales Commission AgreementThe above is only an example. Your agreemernt will differ depending upon terms that you wish to negotiate upon.Hope this helps. All he best!