In order to request payment for pre order items:
Locate the booking and click pre-orders.
Scroll down past the pre order items to total paid and click Add payment.
Add in the percentage of the remaining total that you would like to request.
Click Add.
Select the type of payment from the drop-down box e.g., Request Payment.
Click Add Payment.
Once your customer has made this payment this will appear in both payments and in the pre-orders for their booking.