code: func (r Ticket) GetServerAdministrationBillingInvoice() (resp datatypes.Billing_Invoice, err error) {
  	err = r.Session.DoRequest("SoftLayer_Ticket", "getServerAdministrationBillingInvoice", nil, &r.Options, &resp)
  	return
  }
docs: // Retrieve The invoice associated with a ticket. Only tickets with an associated administrative charge have an invoice.