"pid": "00000000",
"order_number": "{{ orderId }}",
"currency": "JPN",
"items": [
{% for OrderDetail in Order.OrderDetails %}
{
"code": "{{OrderDetail.product_code}}",
"price": {{OrderDetail.price}},
"quantity": {{OrderDetail.quantity}}
}
{% if loop.last == false %},{% endif %}
{% endfor %}
],
"total_price": {{Order.payment_total}}



こんな感じでどうでしょうか？