The customer should be able to connect every invoice amount to the contract, approved changes, and work completed without reconstructing the project history.
Match the contract's billing method
Use milestone billing when the contract does, a schedule of values for progress billing, and itemized labor, material, equipment, and markup for time-and-material work.
Keep change orders traceable
List each approved change by number, date, description, amount, and prior billing. Do not hide unapproved scope inside a generic invoice line.
Show the balance bridge
Present contract value, changes, earned or completed work, retainage, prior invoices, payments, credits, current due, and remaining balance in a consistent order.