A confusing billing entity
A customer’s trading name may differ from its billing name. Use the billing details the customer supplied and double-check the invoice address.
An ambiguous amount
List the currency, quantity and price. “Three hours at 75 USD” is easier to verify than a single unlabeled amount of 225.
A duplicate or missing reference
Assign each issued invoice a unique number. Add the customer’s purchase order or project reference when they use one.
Unclear adjustments
Label discounts and tax distinctly. Compare the final total with the accepted quote, and verify the accounting rules that apply to your transaction.