curl --request GET \
  --url https://api.railmail.app/api/v1/billing/usage \
  --header 'X-API-Key: <api-key>'

{
  "subscribersCurrent": 123,
  "subscribersLimit": 123,
  "subscribersPercent": 123,
  "campaignsCurrentMonth": 123,
  "campaignsLimit": 123,
  "campaignsPercent": 123,
  "projectsCurrent": 123,
  "projectsLimit": 123,
  "aiCreditsCurrentMonth": 123,
  "aiCreditsLimit": 123,
  "aiCreditsPercent": 123,
  "emailsCurrentMonth": 123,
  "emailsLimit": 123,
  "emailsPercent": 123
}