交換 and 返品: exchange and return
Distinguish exchanging an item from returning it.
Exchanging for another item
交換 (こうかん) means exchanging things or replacing one with another. 「このシャツを大きいサイズに交換したいです」 requests an exchange for a larger size. It is a request, not a completed exchange.
「名刺を交換しました」 means exchanging business cards. 交換 is used not just for replacing purchased goods but also for exchanging objects or information.
Returning merchandise
返品 (へんぴん) means returning purchased merchandise to the seller. 「商品を返品しました」 says that the item was returned. It does not explicitly say a replacement was received or a refund processed.
「返品の理由を説明しました」 says that the reason for the return was explained. Giving an explanation does not establish approval of the return.
Asking whether it is possible
「交換できますか」 asks whether an exchange is possible; 「返品できますか」 asks about a return. Distinguish the words' meanings from a shop's actual conditions. Permission for one does not establish permission for the other.
「交換ではなく返品を希望します」 requests a return rather than an exchange. It explicitly distinguishes requesting another item from returning this one.
Does 「返品したいです」 mean a refund has already been received?
Show the answer and why
No. It expresses a wish or request to return the item. The return and refund process must be checked separately.