JSONata

/* JSONata expression to transform an order into an invoice summary */
$sum := function($arr) { $reduce($arr, function($prev, $curr) { $prev + $curr }, 0) };

{
  "invoice": {
    "customer": Account.Name,
    "date": $now(),
    "items": Account.Order.Product.{
      "name": `Product Name`,
      "quantity": Quantity,
      "unit_price": Price,
      "total": Quantity * Price
    },
    "subtotal": $sum(Account.Order.Product.(Quantity * Price)),
    "tax_rate": 0.08,
    "tax": $sum(Account.Order.Product.(Quantity * Price)) * 0.08,
    "grand_total": $sum(Account.Order.Product.(Quantity * Price)) * 1.08,
    "item_count": $count(Account.Order.Product),
    "most_expensive": Account.Order.Product^(>Price)[0].`Product Name`,
    "discount": $sum(Account.Order.Product.(Quantity * Price)) > 500 ? "10% loyalty discount applied" : "No discount"
  }
}