Build the invoice from the line items
A tax-exclusive invoice begins with unit price times quantity. Tax is charged on that subtotal, so invoice total equals subtotal plus the calculated charge. This model assumes every unit is taxable at the same rate and that no separate shipping, discount, or environmental fee changes the taxable base.
Quantity comes before the tax charge
Twelve units at $45 create a $540 subtotal. At 7.5%, the tax is $40.50 and the invoice total is $580.50. The quantity is multiplied before tax is calculated; calculating one rounded tax amount per unit can produce a different cent result under some billing rules.
Quote changes need a new subtotal
A frequent quotation error is calling a per-unit price the invoice total, especially when quantity changes after approval. Another is applying tax to a subtotal after a discount without checking whether the discount was granted before tax or reimbursed later by someone else.
Simple invoices have a narrow boundary
Use this page to build a simple line-item estimate with a supplied rate. It cannot price tiered quantities, choose a jurisdiction, handle tax-exempt lines, or reproduce a supplier's invoice engine. Contract terms and the final invoice decide what is collectible.
Separate the moving parts
For repeat orders, put unit count, unit price, discount basis, tax base, and rate on separate lines. That exposes whether a total moved because more goods were ordered or because the tax treatment changed.
For a purchase order, distinguish an estimated quantity from the quantity actually delivered. Five missing units change the subtotal before any rate is considered, whereas a changed rate changes the charge on the same subtotal. That separation helps a buyer approve a quote without confusing a logistics shortfall with a tax dispute. If freight is separately invoiced, place it in its own line and confirm whether the jurisdiction treats that line as taxable before folding it into the total.
If the supplier rounds per line, mirror that sequence for a realistic check. A subtotal-level estimate remains useful, but should not be presented as the issued invoice.
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