Since GroovePacker's primary function is packing verification we've chosen to make sure that GroovePacker continues scanning regardless of what the inventory levels are.
If you have the correct item in your hand, and are scanning it while GroovePacker has a 0 quantity on hand, it must mean the inventory in GroovePacker has not been kept up to date. No need to slow things down by calling attention to this fact.
If your inventory level reaches 0, and you continue scanning, the count will go into the negatives. The other option here would have been to force it to stop at 0. Since you'll need to update it anyway we just to let it go negative as the negative value will give you an exact count of how many pieces were shipped since inventory dropped below 0.
This additional information may assist in choosing when or how much inventory to order.
GroovePacker makes it easy to update inventory values through the receive and recount feature, directly in the products list, or via CSV import.