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.

All of which is just to say: Ouch!

Be well,
rir