What can the first reply reasonably promise?
Tell the customer that you can help them submit a request. Do not promise eligibility before checking the purchase and the applicable policy. A calm acknowledgement gives them a next step without committing the store to a decision it has not made.
Ask for the relevant order reference and item. If several items are involved, make that explicit rather than assuming the entire order should be returned. Then ask for the reason in the customer's own words or through clear choices with an option to explain.
Avoid treating every request as a product defect. Wrong size, damage, a missing item and a changed preference may need different checks. The first flow should identify the job, not decide every exception.