Subtotal is not calculated correctly at order preview - actions-on-google

Question

I want to create a chatbot with Dialogflow and Google Assistant along with Google Transactions API for enabling a user to order some items. For now my agent contains the following four intents:
Default Welcome Intent (text response: Hello, do you want to buy a chocolate box?)
Default Fallback Intent
Int3 (training phrase: Yes, I want, fulfilment: enabled webhook)
Int4 (event: actions_intent_TRANSACTION_DECISION, fulfilment: enabled webhook)
I am using Dialogflow Json instead of Node.js to connect my agent with Transactions API. I want to build a cart and an order for the user by using finally that the user meets the transaction requirements by using the actions.intent.TRANSACTION_DECISION action of Google actions. For this reason, following Google docs, when Int3 is triggered I am using a webhook which connect Google Assistant my back-end which sends back the following json (to trigger actions.intent.TRANSACTION_DECISION) :
{
"fulfillmentText": "This is your order preview:",
"payload": {
"google": {
"expectUserResponse": true,
"isSsml": false,
"noInputPrompts": [],
"systemIntent": {
"data": {
"#type": "type.googleapis.com/google.actions.v2.TransactionDecisionValueSpec",
"orderOptions": {
"requestDeliveryAddress": true
},
"paymentOptions": {
"actionProvidedOptions": {
"displayName": "VISA **** **** **** 3235",
"paymentType": "PAYMENT_CARD"
}
},
"proposedOrder": {
"cart": {
"lineItems": [
{
"description": "Book",
"id": "1",
"name": "Book",
"price": {
"amount": {
"currencyCode": "USD",
"nanos": 0,
"units": 31
},
"type": "ACTUAL"
},
"quantity": 2,
"subLines": [],
"type": "REGULAR"
}
],
"merchant": {
"id": "Amazon",
"name": "Amazon"
},
"otherItems": []
},
"id": "<UNIQUE_ORDER_ID>",
"otherItems": [
{
"id": "Subtotal",
"name": "Subtotal",
"price": {
"amount": {
"currencyCode": "USD",
"nanos": 0,
"units": 62
},
"type": "ACTUAL"
},
"type": "SUBTOTAL"
},
{
"id": "Delivery fees",
"name": "Delivery fees",
"price": {
"amount": {
"currencyCode": "USD",
"nanos": 0,
"units": 10
},
"type": "ACTUAL"
},
"type": "FEE"
}
],
"totalPrice": {
"amount": {
"currencyCode": "USD",
"units": 72
},
"type": "ACTUAL"
}
}
},
"intent": "actions.intent.TRANSACTION_DECISION"
}
}
}
}
However, I am getting the following errors on Google Assistant simulator:
MalformedResponse
expected_inputs[0].possible_intents[0].input_value_data.transaction_decision_value_spec.proposed_order: subtotal price is not sum of regular lineItems. Expected-> Sum of line item price: units: 31 nanos: 0 currency: USD Actual-> Provided total price: units: 62 nanos: 0 currency: USD.
MalformedResponse
expected_inputs[0].possible_intents[0].input_value_data.transaction_decision_value_spec.proposed_order: total price is not sum of lineItems and otherItems. Expected-> Sum of line item price: units: 42 nanos: 0 currency: USD Actual-> Provided total price: units: 72 nanos: 0 currency: USD.
Why I am getting this error since I have specified that "quantity": 2 in the respective lineItem?
It is very obvious that the subtotal should be units: 62 since I have ordered "quantity": 2 products of units: 31 price each...
The final order preview looks like this (this is borrowed from Google docs examples):

Answer 1

The answer seems to be in the error message: the API expects a each line item's price to be a sum of the total items in that line item instead of a quantity price (relevant error phrase: "Sum of line item price"). The corrected JSON should look something like this:
{
"fulfillmentText": "This is your order preview:",
"payload": {
"google": {
"expectUserResponse": true,
"isSsml": false,
"noInputPrompts": [],
"systemIntent": {
"data": {
"#type": "type.googleapis.com/google.actions.v2.TransactionDecisionValueSpec",
"orderOptions": {
"requestDeliveryAddress": true
},
"paymentOptions": {
"actionProvidedOptions": {
"displayName": "VISA **** **** **** 3235",
"paymentType": "PAYMENT_CARD"
}
},
"proposedOrder": {
"cart": {
"lineItems": [
{
"description": "Book",
"id": "1",
"name": "Book",
"price": {
"amount": {
"currencyCode": "USD",
"nanos": 0,
"units": 62
},
"type": "ACTUAL"
},
"quantity": 2,
"subLines": [],
"type": "REGULAR"
}
],
"merchant": {
"id": "Amazon",
"name": "Amazon"
},
"otherItems": []
},
"id": "<UNIQUE_ORDER_ID>",
"otherItems": [
{
"id": "Subtotal",
"name": "Subtotal",
"price": {
"amount": {
"currencyCode": "USD",
"nanos": 0,
"units": 62
},
"type": "ACTUAL"
},
"type": "SUBTOTAL"
},
{
"id": "Delivery fees",
"name": "Delivery fees",
"price": {
"amount": {
"currencyCode": "USD",
"nanos": 0,
"units": 10
},
"type": "ACTUAL"
},
"type": "FEE"
}
],
"totalPrice": {
"amount": {
"currencyCode": "USD",
"units": 72
},
"type": "ACTUAL"
}
}
},
"intent": "actions.intent.TRANSACTION_DECISION"
}
}
}
}