I have a clothing website where I link to various clothing retailers. I have reached an agreement with one of the retailers whereby they will pay a commission to us for every sale they make from traffic that was referred by our site. I need a mechanism for tracking how much commission should be paid to us, that involves as little work as possible to implement from their side.

We both have Google Analytics.

Option 1: They record a goal in their GA account whenever someone makes a purchase on their site. They see how many completed goals are marked as referral traffic from our site and calculate commission accordingly. The problem with this is that the whole process of calculating and paying commission will be manual. They will need to frequently check how many sales were generated by referral traffic from our site, and probably we will have to chase them for commission payments. Also - since we won't have access to their GA data - we will need to trust that they report all sales accurately.

Option 2: Sign them up to an affiliate network like Commission Junction or Google's Affiliate Network, and connect to them through this network. The problem with this solution is that it seems too heavyweight; ideally we don't want to ask a retailer to go through the whole sign up process just to deal with us and pay us commission.

I am assuming that there must be some lightweight service that tracks the number of sales by one site and pays commission accordingly to the other site, where the sign up and installation procedure is simple and fast.