As a Merchant you should always process a Refund (full or partial) to close out a plan, never a cancellation. Recently the cancel option was removed for most Merchants, but if you still see it, please utilize the refund option instead.
Once a plan is active, fees for any refunds are handled per your Merchant Agreement. Please refer to that agreement for exact details.
You can also partner with your Account Manager to review these details to see how refunds and fees work for your account.