Need to tag my website with e-commerce tag for google analytics

In view to better analyse my customer comportment and how to improve my conversion rates for my website, I need a freelancer to set up the e-commerce tag of google analytics on my website. For payment, my website use a system named [login to view URL] As I sell access to content, this system block the user before he buys when he buy for one day, one month, three month or one year (only 4 subscriptions), he can access to all the content of all my pages.