commerce unique identification number for each item on each transaction
prepend your unique id with the order number. Since it's already unique within the order, and presuming an order number is unique, [order-number]-[unique-id] should return a truly unique value with the added benefit of being traceable since the order number would have a searchable or reference-able value.

EDIT:

Further research yielded the module Field default token 

This module enables to use tokens as field default values.


Using this you should be able to create the order-ID combination (or any other combination). To prevent user tinkering, I would also suggest Field Permissions:

The Field Permissions module allows site administrators to set field-level permissions 
to edit, view and create fields on any entity.