1. Go to Commerce > Auction
  2. Hover over your event and click Bookkeeping
  3. Click Manage Initiative Records
  4. Click the radio button for Won Bid
  5. Click Edit next to the transaction
  6. Click Delete Record From Initiative
  7. Click Delete
  8. Confirm the deletion
This will refund the transaction immediately from Blackbaud's end, but it may depend on the bank/card company how long it takes for the refund to show on the supporters credit card statement.