for (var i = 0; i < selectedItems.length; i++) {

                    window.opener.console.log('Adding Item: ' + selectedItems[i].id);

                    currentTransaction.setCurrentSublistValue({
                        sublistId: 'item',
                        fieldId: 'item',
                        value: selectedItems[i].id,
                    });

                    currentTransaction.setCurrentSublistValue({
                        sublistId: 'item',
                        fieldId: 'quantity',
                        value: selectedItems[i].quantity,
                    });

                    currentTransaction.setCurrentSublistValue({
                        sublistId: 'item',
                        fieldId: 'rate',
                        value: selectedItems[i].rate,
                    });

                    currentTransaction.setCurrentSublistValue({
                        sublistId: 'item',
                        fieldId: 'amount',
                        value: selectedItems[i].amount,
                    });


                    currentTransaction.commitLine({
                        sublistId: 'item'
                    });
                }

pretty straight forward.. just runs through and sets the item. I've tried selecting a new line and just setting on current sublist line and they both produce the same result.