I visited Aldi and Amazon's cashierless grocery stores and both overcharged me. That aside, the Amazon experience felt smoother.

Grocers are testing out cashierless stores in more locations as they seek to cut running costs for their brick-and-mortar operations. These stores use sensors or cameras to monitor what customers pick up from the shelves, and bill them after they leave.

Amazon opened its first European cashierless store in Ealing, west London, in March 2021. The store, which uses Amazon's "Just Walk Out" technology, is known as Amazon Go in the US, but in the UK, it's operated at 17 London locations under the Amazon Fresh brand.

I visited the Amazon Fresh store in Canary Wharf, London's eastern financial hub, which opened in May 2021.

I compared it with Aldi's sole cashierless store, Aldi Shop & Go, which is in Greenwich, east London. The German discount retailer opened the store in January 2022 and I visited it twice soon after. It's 2.5 miles from the Amazon Fresh store in Canary Wharf.

The stores had signs inside and outside explaining to customers how to enter and shop, suggesting they expected visitors to have questions.

I visited the Aldi store a week after it opened. The exterior of the store had a row of digital screens explaining how the store format worked, and inside there were staff members helping people use the app.

The Amazon store also had a member of staff inside the entrance to help customers with queries.

I knew I needed apps to enter both stores, so I downloaded them before leaving home.

To visit the Amazon Fresh store you can use the regular Amazon app. But using it was a bit confusing – it seemed counterintuitive that the QR code to enter the store wasn't under the "Grocery" tab, which was dedicated solely to Amazon Fresh deliveries for Prime users. Instead, I had to click on an item to take me to my basket and then click on "fresh code."

To enter Aldi Shop & Go, you need a dedicated app that seems to have no function other than to get you into the store. But the app itself seemed easier to use than the Amazon one, even if you had to go through a tutorial explaining how the store worked before you could access the QR code.

Both apps generate a QR code that you need to swipe on the entry gates to get in. Then sensors throughout the stores monitor what you take from the shelves.

Both stores had opened within the past 12 months and their decor was minimalistic and modern. It was the first time I'd been in an Amazon store and it was strange seeing its branded merchandise.

Beyond that, the stores looked pretty much like normal grocery stores. The noticeable differences were that people put their shopping straight in their bags rather than in baskets, and there weren't any cashiers, though there were staff dotted around to help.

Both stores had bakery sections with loose pick-and-mix products, including fresh bread, doughnuts, muffins, and pastries.

I thought this was interesting because the bakery products weren't packaged, unlike other items in the stores. I was intrigued to see whether the sensors would be able to identify everything I picked up.

You have to be 18 or over to buy alcohol in the UK but most retailers say they'll ask for ID if you look under 25.

In the Amazon Fresh store, alcohol was available in a small room at the back. There was a member of staff stood at the entrance who'd only let people in if they looked over 25 or could show ID. This was the only part of the store where I didn't spot any Amazon-branded products.

In the Aldi Shop & Go store, alcohol wasn't sectioned off and was instead just stacked at the end of an aisle. There was a sign saying that you had to verify your age either using the app or via a store worker to buy alcohol.

But on my first visit, I didn't see the sign and just walked out of the store with alcohol. It was only after returning home when I opened the app again that I saw a message saying that I needed to have my age verified, which hadn't come up on my phone as a push notification.

In other words, I was able to buy alcohol without my age being verified.

After seeing the message, I tried using the age-verification software on the app, which requires you to take a photo of yourself, but it didn't work.

On my second visit, I tried to use the age-verification software in store but it still didn't work. A member of staff was telling customers the store was having a lot of problems with the software.

Next to the exit gates, a member of staff was asking every customer whether they had alcohol in their bag. After I said yes, the employee asked for my ID.

Aldi declined to comment directly on the alcohol sales issues after Insider flagged them to the company.

Amazon's verification system seemed quite labor-intensive because it required a staff member to constantly stand next to the alcohol section. However, it seemed much more reliable compared to Aldi's age-verification system, which I largely put down to teething problems at a new store.

Relying on customers to be honest about whether they had alcohol in their bags seemed risky, too.

Both stores had separate exit gates next to their entry gates.

I wasn't entirely sure how to exit the Amazon store. There was a green arrow on one of the gates, but it looked as though I had to press a button. As I walked towards it, though, the gate opened.

In the Aldi store, it was clearer that you were meant to just step in front of the exit gates and they would open automatically. But both times I visited this didn't happen for me – probably because I had alcohol in my bag and my ID hadn't been verified on the Shop & Go app – so a member of staff had to to open the gates for me.

From the moment I entered the Shop & Go store, Aldi's app sent me a stream of push notifications. These included information about shopping as a group, a message thanking me for shopping at the store, and an alert saying my receipt was ready.

I only got one notification from Amazon, which came through a short while after leaving the store. But the notification was very useful: It told me how long I'd spent in the store, how many items I'd got, and how much they'd cost in total, all at a glance.

Aldi's notification simply redirected me to the app to view my receipt.

Both stores identified nearly all my purchases correctly, and both got all my bakery purchases right.

At the Aldi store, my receipt was spot-on for my first trip, but on my second trip it had added a tin of sweetcorn I'd picked up then put back down.

Similarly, Amazon charged me for halloumi, a butternut squash, and a beetroot salad I'd held in my hand before deciding not to buy it, and putting it back.

"Just Walk Out technology is highly accurate," an Amazon spokesperson told Insider. "On rare occasions where a customer may find a discrepancy on their receipt, they can request a refund through the app or reach out to customer service via email and a team member will be able to assist them."

Because both apps didn't seem to have a running list of items in my virtual basket, I didn't realize I was being charged for these items until after I left the store.

Aldi's email receipt had information on how to return items but not on how to get a refund for products you didn't actually leave the story with. The app receipt, on the other hand, had a button saying: "Do you need to change anything?" This takes you to a list of everything you bought, where you can say why you wanted a refund.

I applied for a refund of the sweetcorn, and I checked my app around two and a half hours later to find that the existing receipt said that it was still calculating some of the prices, but that it had added a new receipt with all the same items bar the sweetcorn. I was surprised this was listed as a new transaction, rather than the existing one being amended.

I had a better experience getting a refund from Amazon. I was able to quickly request a refund on Amazon's website, and I got an email the next day confirming the refund had been approved. The receipt was quickly amended on the app to include a refund.

On the whole, Amazon's store seemed much more efficient than Aldi's – but I put this down to teething problems at Aldi's store, which was newer, and the company's first cashierless store.

"This store utilizes the very latest in retail technology, offering Aldi's award-winning products and unbeatable prices to customers in a new and innovative way," an Aldi spokesperson told Insider. "We are pleased with how the trial is operating so far and will continue to monitor it closely."

Ultimately, though, I don't plan on going out of my way to return to either store. This is partly down to having to closely monitor receipts after each visit, and because I don't have enough memory on my phone to keep the apps.

Cashierless grocery shopping is fun as a novelty, but I can't see myself making it a part of my daily routine.