By Nicolas Avila · October 10, 2026

Postal-code shipping calculations: why the wrong price drives customers away

Photo: Avila Ops Tecnologia

When shipping looks too expensive on a website, the postal code is almost never the problem. It is the weight and dimensions recorded for the product. The store sends Correios or the carrier the destination postal code, weight, and box dimensions; if a product is listed as 2 kg and 40 cm per side when it weighs 300 g and fits in a small box, the returned rate doubles and customers leave thinking you charge too much.

Postal-code shipping is the first thing customers calculate and the last thing owners check. A hastily created product record gives the wrong rate on every order, and nobody tells you. Checking takes 15 minutes.

How does the calculation work?

Each time someone enters a postal code, the store queries the carrier’s rate table with four pieces of information:

1. Origin postal code (yours) and destination postal code. These determine the distance band and region.
2. Package weight in grams.
3. Box dimensions: length, width, and height.
4. Service: PAC, SEDEX, Jadlog, Loggi, and so on.

From the dimensions, the carrier calculates dimensional weight (length times width times height in centimeters, divided by a factor; 6,000 for Correios). At Correios, when dimensional weight exceeds 5 kg, billing uses the greater of actual and dimensional weight; below that, actual weight applies. Other carriers have their own rules. A large, light box above this threshold is charged as though it were heavy. The guide measuring packaging: the product that costs more unnecessarily covers this in detail.

With more than one product in the cart, the store adds weights and estimates a box. If every product is listed with its individual box, shipping for two items skyrockets.

Where do errors enter?

Six causes cover almost every case:

How do you check in 15 minutes?

1. Take your five bestsellers and weigh each one already packaged on a kitchen scale.
2. Measure each closed box in centimeters.
3. Compare with the store’s records. Correct any differences.
4. Simulate website shipping for three postal codes: one in your city, one in a distant state capital, and one in an inland town in another state.
5. Quote the same three postal codes, with the same weight and box, directly on the Correios website or the rate table your store uses (counter, contract, or shipping intermediary). The amounts must match that table. If they do not, the problem is in the product records or origin postal code.

Repeat with two products in the cart and adjust grouping rules if the rate jumps. High cart shipping costs are among the reasons covered in abandoned carts: the three most common reasons.

What else prevents shipping from driving buyers away?

Show shipping early: a postal-code field on the product page and in the cart prevents surprises at checkout. Display delivery time in business days alongside the price. Always offer an economy option: people who are not in a hurry choose PAC and feel they saved money. Who pays what, and three ways to split the bill, are covered in what shipping is and who pays for what.

What to do now

Run the five-product, three-postal-code test today. If any rate fails to match Correios, your shipping has been driving customers away since your store opened. Get a store live in one day, with postal-code calculations integrated with Correios and carriers, with no sales commission: avilaops.com/loja-virtual or contact us on WhatsApp: wa.me/5517997811471.

Frequently asked questions

Why is my website’s shipping more expensive than Correios?

Almost always because recorded weights or dimensions exceed the actual values, or because a fixed fee is added. Compare your website’s quote with Correios using the same weight and box.

What is dimensional weight?

It is weight calculated from box volume: length times width times height, divided by a carrier factor (6,000 for Correios). At Correios, it only counts above 5 kg; billing then uses the greater of actual and dimensional weight. A large, light box above this threshold is charged as heavy.

Do I need to enter weight and dimensions for every product?

Yes, or the platform uses a default value and shipping is wrong. Weigh and measure the packaged product. For similar products, enter it once and copy.