> ## Documentation Index
> Fetch the complete documentation index at: https://docs.getlago.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Dynamic

<Note>
  Only the `sum` aggregation type is compatible with the `dynamic` charge model.
  If you have a different use case, please reach out to the Lago team for assistance.
</Note>

Select the dynamic charge model if you calculate the price manually or if prices fluctuate during a billing period (e.g., for SMS APIs, AI models, etc.), and you want to apply a specific **total amount** per event using the property `precise_total_amount_cents`.

Let’s take the example of our AI company. You may want to charge a custom price per event. Consider the following list of events:

<Tabs>
  <Tab title="Table view">
    | Event         | Metric             | Unit | Total amount |
    | ------------- | ------------------ | ---- | ------------ |
    | Transaction 1 | Dedicated Instance | 7    | \$0.70       |
    | Transaction 2 | Dedicated Instance | 5    | \$0.55       |
    | Transaction 3 | Dedicated Instance | 10   | \$2.20       |
  </Tab>

  <Tab title="Events payload">
    <CodeGroup>
      ```bash List of events theme={"dark"}
      {
        "events": [
          {
            "transaction_id": "transaction_1",
            "external_subscription_id": "123456789",
            "code": "dedicated_instance",
            "timestamp": 1728554400,
            "precise_total_amount_cents": "70", //event total amount
            "properties": {
              "unit": "7"
            }
          },
          {
            "transaction_id": "transaction_2",
            "external_subscription_id": "123456789",
            "code": "dedicated_instance",
            "timestamp": 1728558000,
            "precise_total_amount_cents": "55", //event total amount
            "properties": {
              "unit": "5"
            }
          },
          {
            "transaction_id": "transaction_3",
            "external_subscription_id": "123456789",
            "code": "dedicated_instance",
            "timestamp": 1728561600,
            "precise_total_amount_cents": "220", //event total amount
            "properties": {
              "unit": "10"
            }
          }
        ]
      }'
      ```
    </CodeGroup>
  </Tab>
</Tabs>

At the end of the period, the invoice will generate **one fee of \$3.435** (see example below).

| Item               | Unit | Unit price | Amount |
| ------------------ | ---- | ---------- | ------ |
| Dedicated Instance | 22   | \$0,156818 | \$3.45 |

To display more detailed information on fees, please refer to the [custom invoice grouping](/guide/invoicing/overview#custom-invoice-grouping) or the [pay in advance](/guide/plans/charges/arrears-vs-advance#charges-paid-in-advance) option.

<Note>
  Only the `standard` & `dynamic` charge models support the [custom invoice grouping](/guide/invoicing/overview#custom-invoice-grouping).
</Note>