There are some sites out there that had bitcoin payment integration but no refund policy integrated unlike fiat. This should be looked into by most startup looking into opening an ecommerce platform for people to pay with bitcoin. Among the thing I look at before making any bitcoi payment into any system is the review from other buyers, followed by security and the policies attached to this platforms. Most people don't endeavour to read about a platfrom policy which is something important when dealing with an ecommerce platform.
There should be a refund policy in case the customer does not like the goods or the goods are reached to him in damaged conditions etc.
However, the main thing is when you pay in bitcoins, you should keep note of the few things.
1- You would send the amount to the designated address carefully as if the address is typed/copy wrongly, the bitcoin will be send to wrong address and can never be recovered or returned.
2- Send the exact amount as shown on the website. Anything send in excess will not be returned.