Affiliate plugin to track sales of product

Hi,

My client is a book publisher and their previous developer installed affiliate with the membership and supporter add ons.

They want to use it to match sales with the appropriate affiliate that referred them. They don't need to work out the commission as they have a sliding rate and they can work that out manually.

Just track that this affiliate referred that sale.

How do we set that up for them. It seems to be tracing clicks ok but not sales.

Thanks