Start with the evidence
Find the unit in the agreement before entering the quantity. Keep a fixed-price package distinct from its internal effort estimate; hours spent on a package are not automatically hours to bill.
Work through the example
Six hours at USD75 produce USD450. Six sessions at the same displayed rate describe a different agreement even though the multiplication is identical. Review the unit label as carefully as the number.
Make the next step clear
Ask a second person to read the quantity, unit and rate aloud together. If the agreed unit changed, retain that change with the draft. Keep uncertain quantities out of a final document until their source is clear.
Keep the source context
Treat an invoice draft as a description of an agreed business event. Keep the source agreement nearby and distinguish document preparation from approval, sending and receiving payment. These are separate steps even when they happen close together.