That's nothing. So many other shops make their cookies last 30 days or more. Why would anyone want to link to Amazon if their cookie last only one day? Is it because they are big and famous and the customers trust them?
I don't see any major change either. I promote Amazon because they are well known and have a LARGE selection. Using their ECS makes it quite easy to display product info.
I was wondering the same thing today. I am sending 10-15 uniques per day to Amazon, but getting barely any orders. If it were a 30 day cookie I am sure I'd be earning something at least.