Liz wanted to return one of the gifts I'd purchased for her from her Amazon wishlist. She tries to only flag items covered by Prime and that are "Fulfilled by Amazon"; if we have to return an item, then those are indicators of free UPS shipping back to Amazon.
But the item we wanted to return today was showing that I'd be paying almost half the purchase price of the item for return shipping -- for a Prime item Fulfilled by Amazon. Huh.
A few quick searches uncovered an interesting workaround: engage in a Live Chat with Amazon Support. I gave the guy my order number, the specific item name, and said I needed to return it. He immediately created a return slip for me and said there would be no shipping charge. In a few minutes, I got an email with the return shipping label. Later this afternoon, I got an email from Amazon confirming the full amount of my refund.
So: if you're not getting the full refund for a return, try starting a Live Chat with Amazon Support, and see if that works for you.