When you give 'qty returned =0 or less than the pickup qty' on the complete return ticket->ticket items, system thinks that those equipment is lost/stolen and then prompt you to 'mark item as lost' and 'charge customer' when you final edit the complete return ticket.
And then the system will display all the lost equipment under Complete Return Ticket ->Misc items.
The lost equipment will be charged with the 'lost rate' mentioned in the pricelist->item->lost rate field or you can also add 'Lost rate' on the misc item screen only when you invoice the 'Complete Return Ticket'.
Go to complete return ticket ->items page->make qty returned 0 or less than the pickup qty
try to final edit the complete return ticket and then choose mark item as lost and charge customer.
All the lost equipment will be displayed on the Compete Return ticket ->Misc Items
Check for the lost item under settings->pricelist->item->lost rate field
Try to invoice complete return ticket and then it will display the lost equipment and bill them.