With FBA, Amazon provides fulfilment and customer service for your orders, including processing customer returns. Amazon uses their own returns policies to determine if an item is eligible for return.
In most cases, customers can request to return an item within 30 days of receiving it. Amazon may make case-by-case exceptions and accept return requests beyond 30 days of receipt.
In most cases, if an item does not arrive at a fulfillment center within 45 days of the return request, Amazon will charge the customer and credit your account for the item.
When Amazon refunds a customer for your FBA order, they debit your seller account for all or part of the value of the refund. In some cases, for example, when an item is lost or damaged when under Amazon’s control, you may be eligible for a reimbursement for refunds on your FBA orders.
You can check more details on these policies for US and EU marketplace