> ## Documentation Index > Fetch the complete documentation index at: https://www.carstoragesoftware.com/docs/llms.txt > Use this file to discover all available pages before exploring further. # Add a line to a plan > Requires `write:billing`. Add a recurring product (e.g. another car's storage at $350/month) to a customer's existing plan. `amountCents` sets a custom price; a matching existing price is reused, otherwise one is created (hidden from the customer shop). Without `confirm`, nothing changes and the response has a plain-language `summary`, the plan `before` and `after`, and a `previewToken`. Calling again with the same arguments plus `confirm: true` and that `previewToken` applies the change. Takes effect with the next bill (no proration, nothing charged now). A stale token (the plan changed since the preview) is refused with 409; preview again. ## OpenAPI ````yaml /api-reference/openapi.json post /api/v1/billing/subscriptions/{id}/add-item openapi: 3.0.0 info: title: Car Storage Software API version: 1.2.0 description: >- Team-scoped REST API for Car Storage Software. Authenticate with a team API key (`Authorization: Bearer ` or `x-api-key: `) created under Settings → Integrations → API. Keys carry granular `read:*` / `write:*` scopes; read never implies write. servers: - url: https://www.carstoragesoftware.com security: [] paths: /api/v1/billing/subscriptions/{id}/add-item: post: tags: - Billing summary: Add a line to a plan description: >- Requires `write:billing`. Add a recurring product (e.g. another car's storage at $350/month) to a customer's existing plan. `amountCents` sets a custom price; a matching existing price is reused, otherwise one is created (hidden from the customer shop). Without `confirm`, nothing changes and the response has a plain-language `summary`, the plan `before` and `after`, and a `previewToken`. Calling again with the same arguments plus `confirm: true` and that `previewToken` applies the change. Takes effect with the next bill (no proration, nothing charged now). A stale token (the plan changed since the preview) is refused with 409; preview again. parameters: - schema: type: string format: uuid description: Subscription id from `list_billing_subscriptions`. required: true description: Subscription id from `list_billing_subscriptions`. name: id in: path requestBody: content: application/json: schema: type: object properties: productId: type: string format: uuid description: Recurring product to add (from `list_billing_products`). amountCents: type: integer minimum: 0 maximum: 10000000 description: >- Price per billing period in cents (e.g. 35000 = $350.00), before tax and processing fees. priceId: type: string format: uuid description: An existing price to use (alternative to `amountCents`). quantity: type: integer minimum: 1 maximum: 100 carId: type: string format: uuid description: Car this line bills for. confirm: type: boolean description: >- Leave out (or false) to preview: nothing changes, and the response has a `summary` and `previewToken`. With `confirm: true` and that `previewToken`, the change is applied. previewToken: type: string maxLength: 100 description: >- From the preview. Required with `confirm: true`; refused if the plan or the change differs from what was previewed. required: - productId responses: '200': description: 'Preview (`applied: false`) or applied change (`applied: true`).' content: application/json: schema: $ref: '#/components/schemas/V1BillingPlanChangeResult' '401': description: Missing, invalid, revoked, or expired API key. content: application/json: schema: $ref: '#/components/schemas/ApiError' '403': description: API key missing required scope, or API disabled for team. content: application/json: schema: $ref: '#/components/schemas/ApiError' '404': description: Resource not found. content: application/json: schema: $ref: '#/components/schemas/ApiError' '409': description: >- Payments aren't set up for this facility (no connected Stripe account). content: application/json: schema: $ref: '#/components/schemas/ApiError' '422': description: Request validation failed. content: application/json: schema: $ref: '#/components/schemas/ApiError' security: - ApiKeyAuth: [] components: schemas: V1BillingPlanChangeResult: type: object properties: data: $ref: '#/components/schemas/V1BillingPlanChange' required: - data ApiError: type: object properties: error: type: string code: type: string details: nullable: true required: - error V1BillingPlanChange: type: object properties: applied: type: boolean description: False for a preview (nothing changed); true once applied. summary: type: string description: Plain-language description to show the user before confirming. action: type: string description: Short form, e.g. `Add Valet at $50.00`. previewToken: type: string description: 'Pass back with `confirm: true` to apply exactly this change.' customerName: type: string currency: type: string interval: type: string description: Billing period, e.g. `month` or `3 months`. nextBillAt: type: string nullable: true description: When the change takes effect (next bill). before: type: object properties: lines: type: array items: type: object properties: lineId: type: string productName: type: string unitAmountCents: type: integer quantity: type: integer carLabel: type: string nullable: true change: type: string enum: - added - removed - price_changed - swapped_in - swapped_out required: - lineId - productName - unitAmountCents - quantity - carLabel totalCents: type: integer required: - lines - totalCents after: type: object properties: lines: type: array items: type: object properties: lineId: type: string productName: type: string unitAmountCents: type: integer quantity: type: integer carLabel: type: string nullable: true change: type: string enum: - added - removed - price_changed - swapped_in - swapped_out required: - lineId - productName - unitAmountCents - quantity - carLabel totalCents: type: integer required: - lines - totalCents newPriceWillBeCreated: type: boolean message: type: string description: Set once applied. required: - applied - summary - action - previewToken - customerName - currency - interval - nextBillAt - before - after - newPriceWillBeCreated securitySchemes: ApiKeyAuth: type: http scheme: bearer bearerFormat: API Key description: >- Team API key from Settings → Integrations. Use `Authorization: Bearer ` or `x-api-key`. ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.