Preview billing estimates for a checkout request without creating any records.

This endpoint runs the billables estimation flow (dry run) to provide a preview of what the billing would look like if a checkout were created and confirmed. No checkout session, activity logs, or other database records are created.

Use Cases:

• Preview pricing before committing to create a checkout session
• Allow customers to see estimated charges before proceeding
• Validate plan and quantity configurations without side effects

Checkout Type Detection: The checkout type is automatically determined based on the provided fields:

• New Subscription (plan_based): Provide plan_id without contract_id
• Amendment: Provide contract_id with product_quantities

Response: Returns simplified billing estimates with:

• current_invoice: Charges for the current billing period
• next_invoice: Upcoming charges for the next period (null if none)