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

[
  {
    "tier": "<string>",
    "name": "<string>",
    "description": "<string>",
    "limits": {
      "maxSubscribers": 123,
      "maxCampaignsPerMonth": 123,
      "maxProjects": 123,
      "maxAiRequestsPerMonth": 123,
      "maxUsers": 123,
      "maxEmailsPerMonth": 123,
      "customFieldsEnabled": true,
      "automationsEnabled": true,
      "advancedSegmentsEnabled": true,
      "customDomainEnabled": true,
      "apiAccessEnabled": true,
      "prioritySupport": true
    },
    "variants": [
      {
        "id": 123,
        "aiCredits": 123,
        "monthlyPriceCents": 123,
        "yearlyPriceCents": 123,
        "baseCurrency": "<string>"
      }
    ],
    "isCurrent": true
  }
]