After following Neverlast's instructions on how to downgrade, you can also request a refund of the professional fee, provided you haven't listed anything yet or used any other services.

I did not use any tools and want to end my Professional selling plan. Will I get a refund?

You are eligible for a Professional selling plan refund if you have not used any Professional tools or benefits after signing up. If you want to end your Professional selling plan, you are eligible for a one-off refund of up to three months, and your refund request needs to be initiated within 90 days from the payment date of the Professional plan selling fees. If you meet this refund criteria, contact Selling Partner Support for assistance with refunds.