Order of operations (where spreadsheets go wrong)
1. Subtotal first
Σ (qty × unit price), line by line, each line rounded to 2 decimals after multiplication — never round unit prices first.
2. Discount before tax
Trade discounts reduce the taxable base. A 5% discount on ₦26M removes ₦1.3M and the VAT you'd have charged on it.
3. Know your VAT base
In Nigeria (7.5%) and Kenya (16%), shipping billed by the seller is usually part of the taxable value. Zero-rated exports charge 0% but still appear on the invoice.
4. Round once, at the end
Compute in full precision, round the final VAT and total (half-up, 2 dp). Rounding per-line then summing can drift by whole naira on large invoices.
Worked example
40 units × ₦650,000 = ₦26,000,000 subtotal.
Less 5% discount → ₦24,700,000. Plus shipping ₦1,200,000 → taxable base ₦25,900,000.
VAT 7.5% = ₦1,942,500. Total due = ₦27,842,500.
Do that by hand for 12 line items and you'll make the case for automation yourself. The "42-second invoice" demos are really demos of never re-deriving this chain manually.
What makes a receipt trustworthy
Sequential invoice numbers, issuer tax ID, itemized lines with the VAT rate shown, and an auditable trail of edits. Digital tools add tamper-evidence (hashes or signed PDFs) so a ₦30M receipt can't be quietly altered after sending — which matters exactly as much as the arithmetic.