FedEx Poor Customer Service

I have been struggling for the pas two weeks to sort out a shipment invoice query. When importing you pay 40% tax on the value of the items you import. The invoice I received does not match up with this standard customs fee. I have never had this problem in the past. I have been struggling for two weeks to get someone to get back to me on this or just assist me in his matter. I'm scared this I will be penalized for not paying this invoice but how can I when it is clearly inaccurate.