The ID of the payment method, as returned by the payment method listing (User / Payment Method / Listing) or the contextual payment options lookup (User / payment options). When it is not empty or null, userId is required.
Choosing balance explicitly requires paymentMethodId: "balance". Omitting this field, or sending an empty or null value, does not explicitly select balance. The system attempts automatic selection in this order: the charge point's corporate account, an applicable subscription, the user's default personal payment method, then the user's default corporate account. Balance is the fallback only when none of these applies.
For OCPI or Hubject roaming EVSEs, an omitted, empty or null payment method is rejected; provide an explicit paymentMethodId.
