The Avis locations in South Africa are franchises and are well-regarded as being some of the most problematic in the network. I had a less than standard experience with them last month where I was hunted down in the airport to be told that, despite my car being okay when I left, they'd 'suddenly' found damage.

£20 could easily be for damage in South Africa considering the weakness of the Rand. While Johannesburg has an excessive number of toll plazas, the toll can be pennies.