You don't need to log into Payfast as a consumer in order to make a payment. It's optional AFAIK.
Simply follow the steps on their site to install their plugin on your wordpress site.
@intrinzic was spot on about PCI-DSS compliance. A full redirect to Payfast for payment is the safest and most secure option and their tutorial will guide you through that.
It's a guess but I think Takealot are running their own payment gateway which would put them in scope for PCI-DSS and they're most likely using Payfast or Paygate as a switch (for acquiring).