Fedex still owes me money

I previously indicated that need assistance with a refund because I paid customs and duties fees twice for a shipment. I suggested that the amount that I paid double could serve to pay for a new shipment.

Fedex assisted me by having the new shipment delivered and I did not have to pay the customs and duties fees for the new shipment. Fedex indicated, "If the amount is more than what you have paid we will therefore request for you to pay in the difference however if the money is less we will then issue a refund".

The customs and duties fee for the first shipment was R4 415.50 and for the second shipment, it was R3 488.68. That means that an amount of nearly R1 000 is still due to me, based on their earlier commitment (and the difference in the fees). I believe the customer care team is aware of this situation, but they seem not to be interested in refunding me the outstanding amount. I find this concerning.