The way it worked for me, was I recieved an e-mail notification that my subscription was about to expire. In that e-mail was a link along with a discount coupon.
I clicked on the link and it took me to a page where I could select the length of subscription that I wanted.
Clicking buy now, displayed the login page.
Do you have a spam blocker? If you do, maybe it blocked the renewal notification. You may have to add *.avast.com to your allowed list.
The selection page is here
http://www.shareit.com/product.html?cart=1&productid=171566