Booking a group holiday on behalf of your friends isn’t a task most people volunteer for. Coordinating dates, finding the best deals and getting everyone’s passport number right can be stressful and time-consuming.But Kelly Miles, 33, always jumps at the chance of booking everything for her group of friends, who regularly travel together.That's because she has found a way to make an extra bit of cash from taking charge of the bookings. Mum-of-two Kelly, from Portsmouth, uses cashback websites when she makes payments for different parts of the trip to earn a percentage of her spending back.The Citizens Advice manager has pocketed £1,000 in the past two years using this trick. While she also uses these cashback websites day-to-day for all types of personal purchases, her biggest wins are when she makes a large booking on behalf of a big group of people.Cashback is where you receive a financial incentive for spending money using a particular payment method, or for buying a specific product or service.Some banks and credit card providers use this to encourage you to spend using their cards or current accounts, for example.But the type of cashback that Kelly earns is from standalone websites, such as Rakuten or TopCashback, which offer free money to customers for carrying out their online shopping.They pay out when you go to them first and click through to the retailer’s website, where you then make the purchase. Kelly Miles, 33, always jumps at the chance of booking everything for her group of friends, who regularly travel together, thanks to her cashback trickFor example, if you wanted to book accommodation through Hotels.com, you could find it on a cashback website and get a percentage of your booking cost back, paid straight to your bank account.You either earn a percentage of your purchase back, typically between 3 per cent and 8 per cent, or a fixed amount depending on the purchase and website you use.The average holidaygoer makes around £203 a year by using cashback services to book their trips, according to data from Rakuten.Website TopCashback says its members make more than £300 on average and that 3,000 have earned more than £10,000 on all cashback purchases.Kelly tries to go on four holidays a year, having jetted to Majorca, Ibiza, Tenerife, Rhodes, Malaga and Portugal in the past few years. She says she has made back hundreds of pounds by booking for everyone else.She says: ‘I started using cashback when I had my kids and split up with their dad. I was a single mum on quite a limited income so I was just looking for any way I could make my money go further. I’ve been doing it for years.‘Now I’m always the lead booker on any trip I’m on. My friends say it’s like a fee for me doing all the organising.’Kelly saves the money she earns and puts it into a Christmas fund for her two children, aged 12 and ten. She says the only drawback for using cashback services is that it can take months for the money to arrive in her account – but that means that cashback from her summer holidays lands perfectly in time for Christmas.This year, she went to Tenerife with two friends and their two children.She decided on a package holiday to the Spanish island which included flights and a hotel stay for five people.Rather than buying each part of the holiday separately, Kelly offered to pay for the lot on her credit card, making sure she went through cashback website Rakuten.Kelly got a flat rate of £68 for the £2,000 trip – which works out at around 3.4 per cent.Her friends paid her back immediately, she says, and Kelly got the cashback a few months later.She says that she makes cashback on the smaller purchases she needs for her holiday, too, which all add up. Kelly, seen enjoying an ice cream at a market during one of her trips, usually makes £50 to £75 a trip and earns around £200 to £300 a year by going on four holidaysFor example, she says she got a great cashback rate on her parking at Luton airport, receiving £8 for the £70 fee, which works out at around 11.4 per cent.She then managed to get 5 per cent cashback using website GetYourGuide to book a waterpark day for her friends and their children. She received £8 after making the £160 booking.So, in total, she managed to earn £84 in cashback from a total spend of £2,230. While she says it’s not a huge sum of money, she made back nearly 19 per cent of her total share of the holiday, which was only £446.She says: ‘It’s actually really simple to do and I think it’s just a clever way that people can maximise their money. It all adds up and could really take that pressure off when every family is so squeezed at the minute.‘I’ve found it can just be that little buffer that you have, whether it's saving for an emergency fund, for example, or saving for your next holiday.’Around half of cashback recipients put their rewards towards a holiday, while 38 per cent put it in their savings account, Rakuten says.You can still shop around for the best deal before using a cashback service.For example, you can continue to use a flight comparison service such as Skyscanner and then search for the same flight via a cashback website.So if you find a cheap flight, you can go to the cashback company’s site to find the airline or a booking partner and complete the booking there.Kelly used booking website Trip.com through Rakuten to secure around 7 per cent cashback on flights for all her friends. She says she usually makes £50 to £75 a trip and earns around £200 to £300 a year by going on four holidays.But it’s not just holidays that Kelly uses to generate free cash.Cashback websites often offer links to comparison websites so you can get the cheapest deals on big purchases such as car insurance or phone contracts. You can then get a percentage back on what you would have spent anyway.Kelly says she was quoted £450 for her car insurance renewal with her current provider. She found a much better deal on comparison website Compare the Market, costing only £250. By making the payment through a cashback site, she made £45 – netting her a total saving of £245 on car insurance.Over the past two years, Kelly estimates she has clawed back around £1,000 in cashback.So what is in it for the companies paying out? Cashback websites earn a fee for ‘introducing’ you to a retailer – you get a small portion of the fee that they are getting.Your account will be linked to the purchase and your shopping data will be used to target you with tailored advertising.Sometimes ‘tracking’ problems can occur where the retailer cannot verify that you were introduced by the cashback company and so they refuse to pay out.If this happens (which is uncommon), it is unlikely you will get any cashback. So, make sure that you choose the right deal for you, regardless of the cashback offer. If you choose a deal solely on the cashback value, you might end up disappointed.Rakuten’s savings expert, Bola Sol, says: ‘People are becoming more money-savvy as the cost of living rises and travel becomes more expensive.‘While everyone else enjoys the trip hassle-free, the organiser is cleverly taking on the admin while quietly earning cashback on every single booking. And with Rakuten, the bigger the purchase, the bigger the reward – sometimes up to 15 per cent savings.’