The creator of an activity has the ability to refund your payment. Once on the page for the activity, the class creator can click Transactions in the top right. There will be a red Refund button to the right of the transaction you'd like to refund.
If there's an error with your payment or refund, contact the Remind Support team to request a refund (including the service fee). Stripe usually processes refunds in 5–10 business days.