To set up PayFast on your Shopify store, you first need to register for a seller PayFast account with the ability to receive payments.
Once your PayFast account has been activated, you will receive your PayFast Merchant ID and Merchant Key via email. You will also be able to see the Merchant ID and Merchant Key by logging into your PayFast account and selecting 'settings' from your dashboard.
Log into your Shopify admin, click Settings > Payments. In the "Accept Credit Cards" section, select "PayFast" from the drop-down menu and enter your PayFast Merchant ID (this is only numbers) and Merchant Key (this is numbers and letters).
To test that everything is working correctly, you can select the tick box to 'Use test mode' when you've added in your PayFast merchant information. This enables you to check out the same way your customers would, but no cards are charged.