If you have Payment in advance/prepayment on a customer order (COA) it is not possible to forward it to a packing list. The user will be notified that the order item cannot be forwarded to the packing slip before a payment has been registered against the customer order (COA). 
The message is seen on the customer order item (COA-item) in the 'Notes' area. 

When the payment is registered, a PAY reference can be viewed on the main level of the customer order (COA) in the Finance area. Click on the icon with arrows pointing downwards besides the "Show payment" to see the PAY reference.

Once the payment has been made, the stop/message on the customer order item (COA-item) disappears.