curl --request GET \
  --url https://api-v2.missioninbox.com/billing/available_plans/ \
  --header 'Authorization: Basic <encoded-value>'

{
  "user_type": "<string>",
  "available_plans": [
    {
      "type": "<string>",
      "name": "<string>",
      "price_id": "<string>",
      "billing_interval": "<string>",
      "is_renewal": true
    }
  ],
  "grace_period_status": "<string>",
  "renewal_only": true,
  "is_in_grace_period": true
}