Unbelievably poor service

Getting a copy of your invoice from a company should be simple. I've never had any problems with this in the past - it's a simple admin exercise. However, I've sent numerous emails to DHL to simply receive a copy of my tax invoices for recent shipments. After several emails and numerous requests I still have not received them. This is not something that will simply be filed, I need these to claim back a significant amount of import VAT. Why is this so difficult for DHL to do? It is completely unacceptable!