Do I have to refund the original delivery cost?

On a full statutory withdrawal, yes. Article 13(1) requires reimbursing all payments received from the consumer, and it names the costs of delivery specifically.

Article 13(2) then caps it. If the customer chose a delivery option more expensive than your cheapest standard one, you only owe the cheapest standard cost, so an express upgrade is not refundable. Set your cheapest standard delivery in the module settings and the cap is applied for you.

Two cases differ. When only part of an order goes back, the delivery service was still used for what the customer kept, so delivery is not refunded by default. When the goods were faulty, wrong or missing, delivery is refunded in full regardless, because that is a remedy rather than a withdrawal.

Every amount is shown with the reason it is included or excluded, so you can see why before you confirm.