Who pays the shipping cost in an item-exchange refund scenario?

Shopify Discussion
1

From what I understand, there are 2 types of refunds which are:

  1. Fully refund:
    1. Buyer refund all items → seller approved → refund money to the buyer.
    2. Buyer refund all items → seller approved → exchange with other items (based on agreement)
  2. Partial Refund:
    • Buyer refunded selected items in the order → seller approved → refund money to the buyer.

*status in ui change once seller approve

My question: if exchange the item instead of refunding the money, who will bear the shipping cost?