Buy a one-time credit top-up.

Charges the default payment method on file when one exists and returns {"status": "charged", "amount_usd": ..., "new_balance_usd": ...}. Otherwise returns {"status": "checkout_required", "checkout_url": ...} for a hosted checkout session so the purchase can complete interactively. Errors: billing_not_enabled when self-service billing is not configured, invalid_amount when the requested amount falls outside the allowed range.