Fix SagePay 'Please redirect your customer' error
When you are using a 3D Secure enabled credit card without using Sagepay Form, you might receive the error "Please redirect your customer to the ACSURL, passing the MD and PaReq".
Currently we support 3D Secure only through SagePay Form (not SagePay, which is also listed in the Shopify admin). You might have to deactivate your existing credit card payment provider to select a new one.
It's an external payment solution that behaves similar to PayPal. Users will input their address details on your Shopify site as part of the checkout process, and then they get forwarded to SagePay to complete their payment.
SagePay only gives this key to you once, on a page that they recommend you print out for future reference. If you don't have this page, you can contact SagePay customer support: http://www.sagepay.com/contact_us
After enabled, try a small charge with the new service to test it.