LUMU · SHOPPING GUIDES
How to compare the total cost of an online purchase
Lumu editorial guide · September 30, 2026
The lowest item price does not always produce the lowest checkout total. Compare the same product and destination, then account for costs and discounts you can actually confirm.
Use one complete calculation
For a single purchase, start with the item price, add shipping, tax and explicit required fees, then subtract discounts that apply to your order. Include the same quantity and accessories in both offers. If you need a membership only for this purchase, record its cost separately rather than treating its discount as free.
Do not compare a total with tax to a price without tax. Enter the same delivery ZIP code at each checkout. Delivery date, seller and return terms can still make two equally priced offers different choices.
A worked example
The following numbers are illustrative, not current retailer prices. Assume both offers describe the same new product and the tax amounts have already been shown at checkout.
| Cost component | Offer A | Offer B |
|---|---|---|
| Item | $100 | $108 |
| Shipping | $12 | $0, confirmed |
| Checkout tax | $8 | $8 |
| Eligible discount | $0 | $5 |
| Total | $120 | $111 |
Offer B has the higher headline price but costs $9 less under these assumptions. If the $5 discount does not apply, its total becomes $116. Check eligibility before using a discounted total.
Handle unknown costs honestly
If shipping or tax is unavailable, label the amount as a subtotal or an incomplete estimate. Unknown shipping is not free shipping. A checkout-dependent fee cannot be assumed to be zero just because it is absent from a search result.
Use a short worksheet for each offer: exact product, seller, condition, quantity, currency, item price, shipping, tax, fees, eligible discount and observation time. Leave an unknown field marked unknown; do not hide it in the total.
Keep conditional offers separate
- Trade-in value depends on the device and acceptance conditions.
- Carrier bill credits may require a plan and a period of service.
- Coupons may require a minimum spend or exclude the seller or item.
- Cashback may arrive later and may not be guaranteed at purchase.
Compare an upfront purchase separately from financing, service plans or trade-ins. An apparent discount can change the obligations attached to the purchase.
Use Lumu to gather comparable references
Search the exact item, inspect the original listing and open the store to finish the calculation. Lumu can label incomplete cost information; it cannot guarantee a checkout amount that the retailer has not provided.