// Get the product ID of the product you want to auto-fulfill
var productId = 123456789;

// Listen for the checkout.order.completed event
$(document).on('checkout.order.completed', function(event, data) {
  // Get the array of items in the order
  var items = data.items;

  // Loop through the items in the order
  for (var i = 0; i < items.length; i++) {
    // Check if the item is the product you want to auto-fulfill
    if (items[i].product_id == productId) {
      // Fulfill the item
      var itemId = items[i].id;
      var fulfillUrl = '/admin/orders/' + data.order_id + '/fulfillments/new?fulfillment[line_items][]=' + itemId;
      window.location.href = fulfillUrl;
    }
  }
});