Count the work around the invoice
Estimate the hours spent on delivery, changes and follow up. Use a consistent hourly cost and avoid counting the same staff cost twice in direct costs and time costs.
Put a cost on waiting
Money tied up in unpaid invoices cannot fund other work. A financing assumption can turn that delay into an estimated cost. Use the rate relevant to your business rather than a borrowed headline rate.
Change the terms before increasing volume
Compare accounts by contribution and by the share of revenue they represent. A useful first action may be a tighter scope, a deposit or clearer payment terms rather than another sale.
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