Exercises: E-commerce

List three checks you should carry out on an online shop before paying, and explain what risk each one prevents.

Explain the difference between paying by card, using a digital mobile wallet, and paying cash on delivery, and state one advantage of each payment method.

The figure shows the workflow of a secure online purchase.

Before the payment, a step marked with a question mark is missing. Describe what checks that step includes and explain why the final step—saving the confirmation and the invoice—is also part of purchasing securely.

Online shops detect when you leave an "abandoned cart" and follow up with reminders and coupons. Explain what tactic they use, how they know you were browsing, and how you can turn this tactic to your advantage.

When paying online, your bank sends a code to your mobile phone to confirm the transaction (3-D Secure). Explain what additional security factor this step adds, and what happens if someone knows your card numbers but does not have your phone.

Explain the differences between buying from a professional seller and a private individual on a second-hand app: warranty, returns, liability if the product is counterfeit. What protections do you lose when buying from a private individual?

Before buying clothes online, it is advisable to read the return conditions. State four details you would check (time limit, who pays for return postage, required product condition, return location) and explain what problem each one prevents.

Many games allow you to buy content (skins, coins, passes) with a single click. Explain how these in-game purchases differ from a standard purchase in a shop, why it is advisable to ask for permission at home, and what you would check regarding saved payment methods.

A website offers trainers worth $120\ \text{€}$ for only $24\ \text{€}$ "today only" and accepts bank transfer in advance as the only payment method. Analyse the warning signs of fraud in this offer, why bank transfer is the riskiest payment method in this case, and what you would do.

A shop shows the price of headphones as $40\ \text{€}$ "excl. VAT". Knowing that the standard rate of VAT is $21\ \%$, calculate the final price you will pay and explain why regulations require displaying the price to consumers with taxes already included.

Explain what a digital invoice is, what minimum information it must contain, and what advantages keeping it offers over a paper receipt when you need to make a warranty claim.

A mobile phone costs $299\ \text{€}$ in a Spanish shop. On a website outside the European Union it appears for $240\ \text{€}$, but when going through customs VAT ($21\ \%$) plus a handling fee of $15\ \text{€}$ may be added. Calculate the true cost of buying from abroad in the worst-case scenario, compare it with the local price, and make a reasoned decision on where to buy.

An app offers a "seven-day free trial" and then charges $8,99\ \text{€}$ per month with an almost invisible notice. Explain what design tactics it uses to make you forget to cancel (dark patterns), how to organise yourself to avoid overpaying (calendar reminder, checking bank statements), and what rights you have if you have already been charged.

An unfamiliar shop has a 4.9-star rating with five hundred generic reviews ("Great!", "Perfect") almost all published in the same month. Explain how to detect paid-for reviews, what signs reveal they are fake, and how much weight you give to ratings when making a purchasing decision.

Compare a debit card linked to a family bank account and a reloadable prepaid card for supervised online purchases by a minor: what limitations each imposes, what happens if the card number is leaked, and which one you would choose to start with, justifying your answer.

For a gift worth $50\ \text{€}$ you can buy at a local neighbourhood shop or from an online retail giant. Compare price, speed, ease of returns, customer service, and local impact, and argue a reasoned decision for this specific case.

A social media channel publishes this ad: "Invest 300,00 € today in our cryptocurrency bot and withdraw 600,00 € in 30 days guaranteed, risk-free."\na) Mathematically demonstrate what would happen if that 100 % monthly return were real: calculate how much money you would accumulate after 12 months by reinvesting the profits, and explain why this calculation proves that it is a pyramid scheme or Ponzi scheme.\nb) Justify the technical and economic difference between investing in a volatile financial asset and a pyramid scheme, pointing out the three red flags present in the message.

Examine these three complaint cases following a purchase:\n1. Marcos buys a keyboard at a brick-and-mortar store in his neighborhood; upon testing it at home, he does not like the feel and returns the next day to take it back, but the store refuses to refund his money and offers him a voucher instead.\n2. Lucía buys headphones online from a website based in Spain, unseals them to test the sound, and, on the eighth day, requests to return them and get her money back under the 14-day right of withdrawal.\n3. Sergio orders a custom hoodie with his name and his band's logo from an online store; three days after receiving it, he asks to return it because he prefers a different size.\na) Rule on which cases the store is legally required to refund the money and which it is not, according to EU consumer regulations, justifying your answer for each scenario.\nb) Justify why the law grants distance buyers protection that it does not grant to in-person buyers, and what technical or commercial reason justifies the exceptions to the right of withdrawal.

When checking out an order of 24,50 € on a web store, the system offers you three payment methods:\n- Option A: Save the card in the store and activate "1-click checkout".\n- Option B: Manually enter the card number, expiration date, and CVV on each purchase, without saving them.\n- Option C: Generate a single-use virtual card linked to a maximum balance of 25,00 € from your bank's app.\na) Analyze the technical difference between storing your banking details directly on the store's servers and using a gateway with "tokenization".\nb) If the store were to suffer a security breach and its database were leaked to the internet, predict the consequences for you in each of the three options.\nc) Justify which option you would choose to balance convenience and cybersecurity, and which two-factor authentication mechanism (SCA / 3D Secure) would come into play.

Your grandmother wants to shop online for the first time and has received this link via messaging: "Professional blender for 14,99 € (previously 89,90 €), offer valid today only", which leads to the website www.chollos-hogar-express.top.\na) Carry out a security audit of the link: identify four technical and legal elements that you should check on that page before entering any data (certificate and protocol, legal notice and tax ID (CIF), contact channels, and available payment gateways).\nb) Predict the specific risks she faces if she enters her bank details on that page.\nc) Design a simple four-step protocol for an elderly member of your family to independently check whether an online store is legitimate before buying.

A restaurant sells a burger on its premises for 10,00 €. On a home delivery platform, the customer pays 12,00 € for that burger, 2,50 € for delivery, and a 0,90 € "service fee". The platform deducts a 30 % commission from the restaurant on the food price and pays the delivery courier a flat 2,20 € per delivery.\na) Calculate the net income the restaurant obtains from the sale on the platform and compare it with the profit it would make by selling it on its own premises.\nb) Calculate the net margin the platform retains for that order after paying the courier.\nc) Based on these data, interpret the impact of this model on small businesses and the job insecurity of couriers, and propose a consumption alternative that maintains food delivery but is fairer to the restaurant.

An online video game puts 500 "exclusive virtual armors" up for sale in NFT format for 40,00 € each. A classmate wants to buy two using his savings, claiming: "Since there are only 500 units, in six months they'll be worth at least 120,00 € each and I'll make 160,00 € net profit".\na) Explain technically to your classmate what he actually acquires when buying an NFT (the record on the blockchain versus the rights to the digital file) and what would happen to his asset if the game developer decides to shut down the servers next year.\nb) Refute his prediction of guaranteed profit using the concepts of liquidity, supply/demand, and lack of bank backing.\nc) Analyze which digital persuasion techniques (such as FOMO or programmed artificial scarcity) the platform uses to entice minors to speculate with digital assets.

Three people search simultaneously for the same hotel room for the same dates and get these prices:\n- User A (desktop computer, 1st visit, accesses through a price comparison site): 78,00 €.\n- User B (high-end smartphone, 4th visit in 48 hours to the same hotel, official app): 98,50 €.\n- User C (mid-range smartphone, 1st visit, incognito tab on a 4G mobile network): 84,00 €.\na) Interpret which digital tracking mechanisms (cookies, device fingerprinting, demand history) explain the 20,50 € difference between User A and User B.\nb) Design a verification experiment with three technical steps to check whether an online store is applying dynamic price discrimination to you.\nc) Justify whether this practice complies with ethical principles and personal data protection legislation in e-commerce.

A student has a fixed income of 50,00 € per month. Attracted by "buy now, pay later (BNPL) at 0 % interest in 3 installments" advertising, they make these three purchases in the same week:\n- Sneakers: 3 installments of 24,00 €.\n- Headphones: 3 installments of 15,00 €.\n- Jacket: 3 installments of 18,00 €.\nThe contract terms apply a flat late-payment penalty of 12,00 € for each unpaid installment.\na) Calculate the monthly total that must be paid during months 1, 2, and 3. Compare that figure with their monthly income and predict what will happen in month 1 if they have no prior savings.\nb) If the service advertises "0 % APR interest", explain what the actual business model of these microcredit platforms is and why they generate a higher risk of over-indebtedness among young people than traditional bank credit.\nc) Design a preventive financial rule (formula or maximum installment percentage relative to disposable income) to avoid defaulting when using this type of service.

Unit challenge. Imagine that you need to apply for your official Youth Card online and buy a pair of headphones for studying. Design a Practical Guide to Security and Digital Services on one page using LibreOffice Writer or Calc.

Follow these three phases:

1. Administrative procedure: Access the electronic headquarters of your autonomous community or the FNMT. Research the steps to obtain the Cl@ve system or the digital certificate. Describe what personal data they require and what legal validity guarantees they provide compared to in-person procedures.

2. E-commerce: Find the headphones in two real online stores. Analyze and compile in a table: presence of an SSL certificate ($https$), secure payment methods, legal cancellation policy (minimum of $14$ days), and additional costs (for example, €$3,50$ for shipping).

3. Reasoned proposal: Write a brief comparison of how user identity is protected on the public portal versus the private store, concluding with three key recommendations to prevent scams and unsafe purchases.

Assessment criteria:

• Correct identification of access and authentication mechanisms in public administration (Cl@ve or digital certificate).

• Rigor in the analysis of security parameters, legality, and payment gateways in e-commerce.

• Technical quality and expository clarity of the comparative document created with LibreOffice.