Suspect charges and refusal to answer queries

I seem to have the exact same issue as other clients. I requested an invoice and had to wait a long time to receive that. When I received it, the VAT charged was nearly double what it needed to be and there was Duty charged that exceeded even that exorbitant VAT on products that do not even attract duty fees. I have been requesting the SAD500 and proof of payments for a week now and all I receive is a notice to make payment.