I'm cancelling a subscription, how can I ensure that the unfulfilled orders alone are refunded?

When a subscription is canceled, all queued orders that have shipping dates in the future get canceled automatically. Since Chargebee does not readily support delivery-based proration, choose Don't issue credits for the unused period while canceling the subscription. Then, do one of the following:

Use the refund order action against each unfulfilled order, choosing the maximum refundable amount that is pre-filled in the dialog. OR
Create a credit note for the invoice associated with the orders, for the amount equal to the total value of the unfulfilled orders. Choose the reason code as Order cancellation to ensure that the refundable credits are associated with the orders that were canceled.

With the help of "The line items' billing period on the invoice" settings, the service period for the line items in the credit notes will be the same as their respective billing periods on the invoice.
Navigate to Settings > Configure Chargebee > Billing LogIQ > Billing & Invoices > Set default service period for credit note line items. Please refer to this link for more information.