Amazon Only partial refund for 3rd party transaction , Internet

Amazon has a 3rd party seller named Kitty Hawk Wholesale.  I bought a toy from them for my grandson for Christmas at the end of November.  They charged me $92.31 and another $12.95 shipping.  When it arrived, it was in a Walmart box with a Walmart receipt showing that they paid $59.99 for it and it was shipped for free.  They never touched this item, never paid to have it shipped, it was never in their possession.  I contacted them to return this item as I ordered it from Walmart myself, and was told I would receive a refund within 5 days of it being returned.  It was shipped back to them, at my expense, on December 8th with a delivery date of December 18th.  As of January 2nd, I had no refund, and started emailing Kitty Hawk Wholesale about when I could expect it.  Every time they responded it was "tomorrow."  I finally filed a claim with Amazon on January 11th because they have their "A to Z" guarantee regarding their 3rd party sellers.  They refunded me $78.46!  Where's the rest of it???  According to the explanation, Kitty Hawk doesn't refund shipping, even though they never paid anything to ship it to me, and I really don't know why I didn't get the rest of it other than I was told I got the refund I "deserved."