Gifts carry two things at once, the thought and the object, and only the first is the point. A receipt makes the second exchangeable without touching the first, and it quietly says you would rather they had something useful than be right about what they liked. The care already happened in the choosing.

Include the receipt without ceremony and say that swapping it is entirely fine. Do not ask later whether they kept it. If you receive one, exchange the thing without guilt, since the giver has already told you that is allowed.