Frustrations at AVIS Cape Town

Two frustrating incidents have left a very bad taste in my mouth regarding Avis Cape Town. The first was resolved (thank you), but the second is still outstanding. After filling up the rental car before returning it, I was charged for petrol when I received the tax invoice. BTW-I have the petrol slip as proof when I filled up if anybody needs it?!?!? This now has to be investigated and will take additional time to resolve. Avis Cape Town -why is it your Durban branch gets everything right but your admin processes are appalling. I need this resolved ASAP as I need to submit invoices to my company