Proforma/Tax Invoice different amounts

I have lost count of how many times this has happened. We receive a ProForma Invoice with notification of an incoming shipment. We pay the amount. No Tax Invoice is delivered with the shipment. We try, at least five times, to get the Tax Invoice (as we cannot claim VAT without it) We get the Tax Invoice, and the amounts do not match. NO matter how many times we ask for explanations, corrections or anything usable, we get no replies. We had one "low cost" letter for a very high cost shipment - how on earth can this be correct??? No follow up on over payments or short payments - so where is the additional money going and who pays in the short fall? On their website, there is link; "Complaint? Compliment? Go Straight to the top and tell John Lucas Country Manager, DHL Express South Africa. Contact John Lucas" Well, do not bother. I have YET to receive any replies to my emails. Jackie Munday has yet to reply to my query sent several times since 20th April....