Good day. I don't see JoomShopping listed on the PayFast website: https://www.payfast.co.za/shopping-carts/
I have contacted them and they are not aware of JoomShopping either.
Are you sure your payment plugin, https://www.webdesigner-profi.de/joomla-webdesign/shop/payments/payfast.html?lang=en is compatible with PayFast in South Africa?
Joomla: 3.5
JoomShopping: 4
JoomShopping: 4