Why orders split
One order, three sources. Part of it ships from your shelf, part from one distributor, part from another, each on its own day.
Where the spreadsheet comes in
Tracking which line shipped from where, and what's still open, usually ends up in a spreadsheet beside the order. It's out of date as soon as a tracking number changes.
Keep the split on the order
Route each line to a source on the order itself, raise the POs from there, and let each shipment record what it carried. The order shows what's shipped and what's open at a glance.
Bill what shipped
Invoice each shipment as it goes, or hold until the order is complete, per customer. Either way, the invoice comes from what actually shipped, not from the original quote.