Hi there Sean!
Thank you for reaching out to WooCommerce.com Support!
If I turn Sandbox mode on, then do a checkout, it goes to https://sandbox.payfast.co.za/eng/process?aff=woo-free which gives a 400 error.
Have you created a new set of sandbox credentials for your Payfast account? These are different to the Live keys that Payfast uses.
To do so, please log in on the Sandbox site:
https://sandbox.payfast.co.za/
The site will create a new set of credentials for you, for use purely for the Sandbox environment.
You can add them via the PayFast module within the Payments tab of WooCommerce:
**WooCommerce** > **Settings** > **Payments** > **PayFast**
Once you’ve added the credentials to the PayFast payments settings, you will be able to test a transaction, which will look similar to this when you click on the “Place Order” button:

Full Size: https://d.pr/i/sHfcdo
Let us know if we can help with anything else!
Regards,
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This reply was modified 2 years, 9 months ago by
Sven L. a11n. Reason: Added a screenshot
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This reply was modified 2 years, 9 months ago by
Jan Dembowski.