how much does Amazon charge sellers for returned products?

How much do sellers get charged when customers return goods? Does Amazon charge the seller the full cost of UPS/usps from customer back to the warehouse? Or is it a fixed fee as part of FBA? As buyers, there are times the seller just let’s me keep the item and sends a new product or refund me. But there are other times the seller still wants me to send the product back before getting a refund/exchange.