Invoices with zero value

We make invoices where the amount = 0. We do this when we deliver a sample product, documentation and so on. In the Cust. Ledger entry table, these records will never end up ‘closed’ with a ‘Closed by Entry No.’ and a ‘Closed by Date’, because we won’t receive any payment on these invoices. Does anyone know if this affects any statistics based on the Cust. Ledger Entries, for example the ‘Average Collection Period’ in the Customer Entry Statistics? And is there a way to get these records closed?