There are 2 approaches for the winner to complete the order:

From Email notification

1. When an auction ends, a winning email notification will be sent to the winner’s email address
2. The winner clicks on Complete your purchase button on the email
3. He will be directed to the Shopify checkout screen

From Auction history

1. The winner logs in his customer account > go to his account page on the Shopify store
2. Check Auction history > Click View button of the auction
3. Click Checkout