If you have a Cause account, you can create a subscription with a custom amount using a Pay Now button as follows:
Log in to your PayFast account.
Navigate to Settings > Generate “Pay Now” Buttons.
Under Text on button, from the drop-down list select Subscribe Now.
Select the button colour scheme, button size and button shape.
Under Button information, enter the item name/service and description.
Select the “buyers can submit custom amounts”. Do not enter any amount.
Enter the Return URL, Cancel URL and Notify URL.
Set the initial amount (the amount that will be debited for the first payment)
Set the recurring amount (the recurring amount that will be debited).
Set the subscription cycles (the number of times that the subscription will run).
Set the frequency of how often the client will be billed (monthly, quarterly, bi-annually or annually)
Define the return and cancel URL
Click Generate Pay Now code.
Copy and paste the HTML code on your website.