Amazon return shipping payment *****

I bought a product from Amazon, but wasn't happy with the quality so I returned it. I was promised that if I emailed them my shipping receipt, they will refund my shipping payment as well. So I returned it and emailed them the receipt. I got the product refund, but not the shipping refund. I contacted them and they told me that the payment was to high. Not one of the agents told me that there was a limit to the shipping payment. So I lost $251.40 (about R3600) and not 1 out of almost 10 support agents were able to help me. I will never buy from them again.