You can receive items in one unit and sell them in another unit, however the receive unit and the sale unit must always be the same. As such, there is no way to setup an item for two selling units. You would have to keep it as a single unit and enter a quantity of 8 when they buy the whole sachet.

The only other option is to create a container, where the container "contains" 8 of the item.  You can then charge a different price for the bundle of 8 vs 8 single items.