The freight-in concept does not apply to non-inventory items, because Manager does not calculate average cost for them. The point of treating freight-in for inventory items in a different way is to include the costs in the cost of goods sold.
Non-inventory items you purchase are simply recorded as expenses. The only reason to define them as non-inventory items is that you purchase them frequently. An example might be copy paper. The paper might be purchased every week, but is not held for sale or production, so it is not considered inventory. The cost of freight-in for non-inventory items is simply included on the purchase transaction (payment or purchase invoice). You can add it to the unit price or separate it as a unique line item posted to a freight expense account if you want to track the shipping expenses separately.