Is replacing purchased clothes (due to unsuitable size or dislike) in stores that allow their replacement with another item of the same value, or a refund, or obtaining a store credit note of the same value valid for a certain period, considered a form of usury (riba)?

This act is not considered usury. Rather, it is a cancellation if the seller pays you the price willingly, or a sale if it is an exchange of clothes for clothes, also with his consent. There is no religious prohibition in either of these two cases. Similarly, if he gives you a receipt for the value of the clothes to buy with within six months, there is no harm in that, provided that he returns the money to you if you do not make a purchase. However, if he keeps the money for himself, then that is not permissible.