Usage Unit VS Qty/Unit

Hi,

I'm selling dog and cat food in bag of 18kg, 8kg and 4 kg.

In Items and Inventory I set up a Kg unit... but I'm not selling unpackaged food, I buy them in bag (18, 8, 4). If I use a Kg unit, then the quantity field in sales order will be in Kg... and not in bag...

My concern is this... I use vTiger and there I use kg so I can know the weight of the order.
vTiger has a "Usage unit" which is kg for me and a "Qty/Unit" which is 18.00 for a 18 kg bag.

Since I'm planning to sync vTiger with Frontaccounting how should I manage this situation?