answer:You can send a thank simply saying thank you for the thoughtful/generous gift. If she is the type of person who really cares to know if the gift worked out, you write a slightly longer note thank her for the generous gift, and let her know that since you already had a mixer you exchanged it for a blender you wanted, and hope she will be over to visit soon for some margaritas. If she will never visit there is really no reason to tell her you exchanged it though. When I send a gift, the reason I want a thank you card or call, is to be sure the intended recipient actually received it. I don’t want my money spent and the gift never to have been received. If the person I sent it to exchanges it, doesn’t matter at all, as long as they have what they need and want. Especially for a wedding, that is why there are registries.