Unit conversion

I am trying to implement erpnext at our organization.
we have a product - welding rods - it comes in boxes and sells in boxes. (boxes containing multiple rods; lets say 1 box = 10 rods)
but when customer demands we open the box and sell them 5 rods.
creating separate item might not be possible as there are many items.
is it possible to convert 1 box to 5 rods while creating sale invoice while price updating accordingly.
i cant figure out to work this around in erpnext.

There is a good article on repack and repack entry. It should help you.