Sagepay documentation says it is possible to use recurring payments and to do that you have to have CPA from your bank. How do I set frequency of payments, and the initial amount, which is going to be different from monthly payments?
The scheduling of Repeat payments is not controlled by Sage Pay. This is something which would need to be developed into the backend of the merchants website.
To perform the Repeat you will be required to reference each repeat request with transaction information relating to the initial authorisation. Take a look at the below guide for more information:
If you are using PI then the process is slightly different: