Shopping Math
How to Compare the Real Price of a Sale
Online stores are good at making a price look simple: 20% off, limited-time deal, free shipping, coupon applied. Real shopping math is more careful. A smaller discount can beat a larger one once tax, shipping, minimum purchase rules, or service fees are included.
Start with the sale price
The discount calculation itself is straightforward. Multiply the original price by the discount rate, subtract the savings, then continue from the discounted subtotal.
If a $125 item is 20% off, the discount is $25 and the sale price is $100. But that is not necessarily the final cost.
Add the costs that appear later
FTC consumer guidance recommends comparing the total cost, including shipping, handling, delivery, taxes, or other fees. That advice matters because checkout screens often reveal costs after the shopper has already committed time to the purchase path.
A practical comparison should include shipping, estimated sales tax, service fees, marketplace fees, delivery fees, and any required add-ons. If a return is likely, return shipping or restocking fees also belong in the decision.
Compare unit price when quantities differ
Two offers are hard to compare when package sizes are different. A two-pack, refill size, subscription price, or bundle can look cheaper while costing more per unit. Divide the final total by the number of usable units to compare fairly.
For groceries, household goods, batteries, toiletries, and subscriptions, unit price often tells the truth better than the headline discount.
A realistic example
Option A lists a $125 item at 20% off with free shipping and 8% sales tax. The sale price is $100, and tax brings the total to $108. Option B lists the same item at 15% off, plus $4.99 shipping, $8.90 estimated tax, and a $1.50 fee. The final total is $121.64. The bigger-looking deal is clearly Option A.
Now imagine Option B is a two-pack. The total may be higher, but the unit price could be lower. That is why the best comparison depends on what you actually need.
A better deal checklist
- Confirm the model number, size, color, condition, and included accessories.
- Check whether the item is new, refurbished, open-box, vintage, or close-out.
- Include shipping, taxes, and fees before deciding.
- Compare unit price for bundles or multipacks.
- Read return rules before buying expensive or hard-to-fit items.
Sources
购物数学
如何比较促销后的真实价格
网店很擅长把价格包装得很简单：20% off、限时优惠、免运费、优惠券已应用。但真正的购物计算要更谨慎。税费、运费、最低消费门槛或服务费加入后，小折扣反而可能比大折扣更划算。
先算折后价
折扣本身的计算很直接：用原价乘以折扣比例得到节省金额，再从原价中减去节省金额。
如果一件 125 美元商品打 8 折，节省 25 美元，折后价为 100 美元。但这还不是最终成本。
把后面才出现的费用加进去
FTC 消费者建议中强调，比较购物时应查看总成本，包括运费、处理费、配送费、税费和其他费用。这个建议很实际，因为很多费用会在结账后半段才出现。
更好的比较方式应该加入运费、预估销售税、服务费、平台费用、配送费和强制附加项。如果商品很可能退货，还应考虑退货运费或 restocking fee。
数量不同时看单位价格
如果包装数量不同，只看总价很容易误判。双件装、补充装、订阅价或套装看起来更贵，但每单位可能更便宜。把最终总价除以可用数量，才能公平比较。
对于食品、日用品、电池、洗护用品和订阅服务，单位价格通常比醒目的折扣数字更接近真实价值。
一个现实示例
方案 A：125 美元商品打 8 折，免运费，销售税 8%。折后价 100 美元，含税总价 108 美元。方案 B：同一商品打 85 折，另有 4.99 美元运费、8.90 美元预估税和 1.50 美元服务费，最终总价 121.64 美元。表面上都是优惠，但方案 A 明显更便宜。
如果方案 B 是双件装，情况又不同了：总价更高，但单位价格可能更低。所以最好的比较方式取决于你实际需要多少。
更可靠的比价清单
- 确认型号、尺寸、颜色、商品状态和包含配件。
- 查看商品是全新、翻新、开箱、旧款还是清仓。
- 决策前加入运费、税费和服务费。
- 套装或多件装要比较单位价格。
- 购买高价或容易不合适的商品前，先看退货规则。