[FIX] missing quantity parameter from get_price_from_picking

The quantity parameter was added to the get_price_from_picking function in commit 65cd88d2167fe855ba941bedafe903c791883bc8 and the delivery.stock.py file was not updated to use the new parameter.

Example usage of this parameter just sums the order line quantities so that is what I have added here, however this mixes units of measurements and so I'm not sure if it is the best approach....