Splitting an invoice's lot-shipping charge so that it may be charged to accounts associated with specific line items is a task usually left to human judgement. There is no reason to imagine that
items belong to the same freight-rate class,
items are shipped from the same location,
an item's shipping weight is consistent
Aside from the above, freight may be put into the cost of the line items or to a general ledger account, like Inbound Freight; how formally this policy is determined can vary.