If you treat as an operation in manufacturing, then you would assume that this is a direct cost to the manufacturing - e.g. depends on the number of products being produced. It's reasonable, because it's not the same pick/pack effort to manufacture 100 than 1000 in a manufacturing order.

If you consider that the pick/pack is not really depending on the volume being manufactured, then I would understand that you are starting to understand it as an indirect cost to the manufacturing process. In that case you could capture the cost as it is produced (each time a transfer is completed you record the cost), and then you would need to allocate the cost afterwards to each manufacturing order according to some cost distribution criteria. 

If you were able to capture, in each pick/pack transfer, what manufacture is it for, then you would be able to do this cost allocation easily.