Do you know why they ask you for your postcode in stores?

It has almost certainly happened to you: you arrive at the checkout of a certain DIY or furniture chain and the sales assistant asks you for your postcode before proceeding to take payment. The truth is that a few people are taken by surprise at such a request, but still end up providing the information without stopping to wonder why the company in question needs it. In fact, since the process has become so customary in major retail stores in Spain, many of us just treat it as part of the normal payment process, taking out our credit card while standing ready with that five-digit postcode on the tip of our tongue.

There are several reasons why staff in the stores ask us for our postcode, some more legitimate than others. Most usually it is to collect demographic data for further market research. Shops can then determine which district, city or region the majority of their customers come from, how much they tend to spend depending on where they live and whether advertising campaigns in certain locations are having the desired effect on sales.

In the first two cases, knowing customer volumes from a demographic point of view makes it possible to make fundamental decisions for large multinationals: is there a real demand for their products and services in nearby towns? Do their inhabitants tend to spend enough to make the potential surface area profitable in the short term?

In addition, by associating postcodes with sales tickets, it can be worked out which products are most popular in certain areas, making it easier to adjust inventories to prevent stock-outs or to avoid unnecessary stock build-ups in warehouses.

On other occasions, as mentioned above, requesting the postcode is fully justified and in the customer's own interests - for example, they must provide it when agreeing to register for the store's loyalty programme so that they can be sent offers specific to their preferred store. It is also essential when requesting home delivery, which incidentally certifies that you live within the previously established delivery area.

Finally, although less and less common due to the rise of mobile payments completed with biometric identification methods, this requirement of the postcode can be an anti-fraud mechanism. Some physical stores have dataphones associated with AVS (address verification system), which checks the postcode provided by the customer against the postcode of the credit card holder. If there is no match then the transaction is denied on the grounds of possible card or identity theft.

By themselves postcodes are not very sensitive personal data, but this fact does not mean that we do not have the right to refuse to provide it. If we do, however, the shops are obliged to ensure that this information is stored anonymously and securely and that it is not shared with third parties unless we give our express consent.

Just recently a post on TikTok by criminologist and cybersecurity expert María Aperador has gone viral. She offers the following recommendation to those who are reluctant to refuse the postcode request from any shop assistant: "I memorised a postcode in Extremadura and I always give the same one. The protection and privacy of everyone starts with small details like this."

The OCU, a national association of consumers and users in Spain, has also expressed its view on the matter, warning about receiving targeted advertising and the sale of personal information as direct consequences of providing postcodes. It recommends not doing so unless it is strictly necessary (this is not usually the case when paying in cash or by card). Also, to consult the store's privacy policy to find out for sure what it will do with the data we provide. Lastly, to be wary of any request we find suspicious, especially if we find ourselves in a retail store we do not usually frequent.

For its part, Spain's leading, independent consumer watchdog Facua reminds consumers that everyone has the right to access, rectify, cancel and oppose the use of their personal data. It advises the public to ask for explanations when a shop is not transparent with their use of the information we are asked to give them, as well as to complain to the data protection authorities if we receive "no answer".