Amounts, Taxes, Discounts & Price Integrity

Never accept the amount from the client — compute it on the server from the cart, prices, discounts, shipping and tax (use a tax engine or the PSP's tax product for multi-jurisdiction sales). Store a price snapshot on the order (unit prices, discounts, tax lines, totals) so later price changes don't alter history. Apply rounding per line or per total consistently with tax rules, and show customers the same numbers you charge.

Price integrity

Python

async def quote(cart: Cart, customer: Customer) -> Quote:
    lines = []
    for item in cart.items:
        product = await catalog.get(item.product_id)              # server price, not client price
        lines.append(Line(product.id, item.qty, product.unit_price_minor))
    discount = await promotions.apply(cart.promo_code, lines, customer)   # validated, limited
    tax = await tax_engine.calculate(lines, discount, ship_to=customer.address)
    return Quote(lines=lines, discount=discount, tax=tax,
                 total_minor=sum(l.total for l in lines) - discount.amount + tax.amount,
                 currency=cart.currency, expires_at=utcnow() + timedelta(minutes=30))

Common pitfalls

• Trusting hidden form fields for prices.
• Recalculating historic orders with current prices.
• Mismatch between displayed and charged totals.

Key points

• Server computes amounts
• Immutable price snapshots
• Consistent tax and rounding

Further reading: Stripe Tax