My opinion with QVC is that it is better to return the item you want to exchange outright for a refund and place a new order for the different size, color etc. In the nearly 25 years I've shopped with QVC their system just seems to work better when items are dealt with independently. Doing it this way doesn't effect the amount of your refund or what you end up spending. Unlike some merchants who ship exchanges for free, QVC does charge you a second time for shipping a new order if you want to change color or size.
They only waive the exchange s&h if they are at fault for you getting the wrong color or size. For example an entire order consisting of three pieces which I ordered recently was incorrect. They offered to take back the return (with no penalty for me using the easy returns label to ship back the incorrect items), and no additional shipping costs for sending the correct items if they got my size back in stock.