Overcharged by FedEx

I am presently having a terrible experience with FedEx. I paid customs and duties when my package was dispatched. Subsequently, my parcel was detained for further checking by customs control when it arrived in SA. FedEx thereafter sent me a sloppy email stating that CUSTOMS had gotten back to them, thus I will be charged x 10 my initial customs cost. It is understandable that my customs duties may be more or less off but to be asked x10 what I had previously paid without a conclusive reason of how we got to these numbers extends beyond unprofessional and borders on ******ion. I would like to speak to someone who is certain of what they are doing please, the agent that I have been dealing with has been on very little use.