curl --request POST \
  --url https://api.salesbricks.com/api/v2/subscriptions/{subscription_id}/upgrade/estimate \
  --header 'Content-Type: application/json' \
  --header 'X-SALESBRICKS-KEY: <api-key>' \
  --data '
{
  "starts_at": "2023-12-25",
  "bricks": [
    {
      "brick_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
      "quantity": 123
    }
  ],
  "plan_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "discount_coupons": [
    "<string>"
  ]
}
'

{
  "ends_at": "2023-12-25",
  "sub_total": 123,
  "grand_total": 123,
  "line_items": [
    {
      "quantity": 123,
      "brick_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
      "brick_name": "<string>",
      "sub_total": 123,
      "plan_id": "<string>",
      "plan_name": "<string>",
      "grand_total": "<string>",
      "grand_total_details": "<string>",
      "tiers_breakdown": [
        {
          "quantity": 0,
          "unit_price": "<string>",
          "sub_total": "<string>",
          "duration_text": "<string>"
        }
      ],
      "pre_commitment_schedule": "<string>"
    }
  ],
  "billing_schedule": [
    {
      "starts_at": "<string>",
      "ends_at": "<string>",
      "sub_total": 123,
      "grand_total": "<string>",
      "grand_total_details": "<string>"
    }
  ],
  "grand_total_details": "<unknown>"
}