Sage Pay is only available as a remote checkout option, so you will not be able to follow along with the guide if you need to use Sage Pay.
The PowerStore checkout is designed to work with a local checkout or Express Checkout from PayPal. When you apply Sage Pay there are three parts to the checkout.
Get Notification Payer Profile
Process Notification Transaction
You could replace the Express Checkout server behaviors with these for Sage Pay. Here is the order you would do this in:
Replace the Authentication for PayPal Express Checkout on the checkout page with the Notification Authentication server behavior.
On the pp_confirm page you will want to replace the 'Get Payer Profile from PayPal Express Checkout' with the 'Get Notification Payer Profile'.
The last one you would want to replace is the 'Process Transaction with PayPal Express Checkout' with the 'Process Notification Transaction' server behavior.
After you have made these updates you would then need to update the set session value server behaviors that are on the pp_confirm page. They are setup to work with the PayPal values, you will need to update these to work with the details that are returned from Sage Pay with the 'Get Notification Payer Profile'. These values should be available to you from the bindings tab once the server behavior has been applied.