You’ve put your shopping in the basket, visited the checkout and handed over your payment – so it might be a bit of a surprise if the cost of your groceries later goes up. That’s exactly what can happen when you shop online with some of the big grocery chains. You may not have realised it, but at some supermarkets the prices shown online when you check out your trolley are “guide prices” that can not be relied upon. At a time when the UK is experiencing record high levels of food price inflation, not knowing how much you are actually going to have to pay for your groceries until after you have agreed to buy them may be a worrying prospect. Estimated prices One Sainsbury’s customer who uses a wheelchair and shops online wrote on Twitter recently that “sneakily increasing” prices at the point of delivery feels “discriminatory”. “I’m disabled, I can’t do a full weekly shop in store,” she tweeted. “When I check out online, for a set price, I expect that to be what I actually pay, not for them to go up literally overnight.” Her case highlights a detail in Sainsbury’s terms and conditions that allows it to change prices before delivery. It’s something about which its customers, and those of some other retailers, need to be aware. “When doing a supermarket shop online, customers are often given an estimated price for fresh produce – such as fruit and vegetables – and then charged for what the item costs when weighed in store on the day,” says Adam French, a consumer rights expert at Which?. He points out that if items cost significantly more than expected when shopping online or you receive substitutions which aren’t suitable, you can opt to return them on the doorstep. “It’s essential that supermarkets are upfront about their terms and conditions and if prices or special offers could change on the day of delivery,” he adds. Last month Morrisons started applying an extra 5% charge to online orders, explaining to customers that this is a “payment deposit” to “allow for any changes at delivery or collection that could alter your order total”. This is added to your card after the checkout price is calculated and not included in the total you are shown. What the supermarkets charge When contacted by Guardian Money, Sainsbury’s, Tesco and Waitrose all said they displayed only guide prices online. It is only after “loose weight” items, typically fresh food, are actually chosen, weighed and delivered that the “final amount” of an online customer’s basket can be accurately calculated, the supermarkets say. However, the variation in the final amount you pay can also reflect changes in the prices of goods you are buying that have occurred between the day of ordering and the day of delivery. Prices may also be affected by promotions, which are valid when you place the order but expire before the date of delivery, or substitutions for unavailable items, depending on the policy of the supermarket. The food poverty campaigner Jack Monroe said on social media that changing online prices “feels obscene” to her: “They [supermarkets] should honour the price at the point of ordering – surely the price factors in whether you order something or not?” she wrote. Tesco states on its website that its prices are “guide prices only” and the actual prices you pay will be those charged in store at the time your order is picked for delivery. As a result, prices can vary “marginally” between checkout and delivery, a spokesperson said. Sainsbury’s makes a similar statement about showing only guide prices online in its terms and conditions, describing the prices you see in “my basket” as “estimates only.” Both supermarkets told Guardian Money that the price online shoppers pay is the same they would pay if they shopped in-store on the day of delivery. Sainsbury’s added that because prices are regularly reviewed, “customers can end up paying less, too.” Waitrose also states online in its terms and conditions that “prices quoted are always estimated … you will always be charged the in-store price on the day of delivery or collection.” However, the supermarket told us that promotional price changes are “typically updated” in its online system in advance, and the website will use the date of your delivery to show you the “most accurate prices”. This means that if you choose a delivery slot at Waitrose before you start to fill your trolley, you may find you are shown more “accurate” prices and the correct “special offers” for the items you are selecting than if you choose a delivery slot later, after filling your trolley. Either way, the supermarket will attempt to anticipate and factor in promotional price changes which have not yet happened in store – but will by the date of your delivery – before you check out. Ocado, Morrisons and Asda use online guide prices, but only for products with a variable weight – for example fresh meat, or vegetables if you have chosen how many you want. So if the product you end up receiving in your delivery has a higher than average weight, you may be charged more than the guide price given online at the time of your order. Equally, if the product you receive weighs less than average, you will be charged less. Retailers’ response In March, Morrisons introduced a temporary 5% holding charge, which is levied on the day before delivery, while the supermarket is waiting to calculate the final total you must pay for these variable weight products, plus any extra charges for substitutions. It will eventually release any money you don’t spend. The total price that you see at the end of the checkout process reflects only the prices of the products you are buying, and does not include the temporary 5% charge, although a spokesperson says customers are informed of it in advance. Morrisons says: “After you can no longer edit your order the day before your order is due, we will attempt to authorise your bank for this amount. This means your bank will hold the money and you can’t spend it. We will calculate the final total after delivery or collection and only charge you that amount.” Other than for loose weight products and substitutions that cost more than the original product you ordered, the price you pay at Morrisons is what you see on the website when you add it to your basket and check out. “If prices change between checkout and delivery, then the customer is not charged an increased price – they pay the price of the item at checkout,” the spokesperson says. Asda said that, apart from loose items priced by weight, “we don’t change our prices once an order has been placed [and] we don’t operate a holding charge of any kind. We do this as we know that a lot of our customers budget very carefully and it would be frustrating for prices to change once an order has already been placed.” An Ocado spokesperson says it does not apply a holding charge either and, apart from loose weight items, “what you see in your basket is what you pay”. All the supermarkets contacted by Guardian Money say that customers receive their receipt with the final prices before their orders arrive and items can always be returned to the delivery driver for a refund.