The British organisation looked at prices of holiday extras, such as car hire, offered by seven airlines flying from Britain to Europe and compared these to the quotes it received by shopping around.
It said that, in some cases, passengers were paying more for inferior products.
For example, when booking a flight from Newcastle to Malaga with easyJet, it was offered car hire for £119 with Europcar, which was £73 more than a deal Which? was offered with Centauro, a car hire broker it rated more highly.
When booking a British Airways flight from Heathrow to Larnaca it was quoted £126 for a week's rental with Avis, compared to £75 with Which? recommended provider Enterprise.
In response, British Airways said: "We offer a range of convenient options, including car hire, to allow customers to tailor their trip to suit their needs. Prices are likely to differ between different car hire companies, and without providing specific details we can't verify that a fair comparison was made."
When Which? added travel insurance to a Jet2 flight from Manchester to Faro, it was quoted £17.50. While the price was similar to a standalone policy with Staysure, Jet2's policy only offered £1,000 for loss baggage and belongings - £500 less than the minimum recommend by Which?. It also pointed out that the Jet2 policy had a high excess of £175.
Wizz Air's add-on insurance with Chubb has a maximum of up to £500 for cancellation and curtailment plus flight cost, which falls short of the £3,000 advised by Which?
"Our advice is to shop around and not be automatically seduced by the easy option when it comes to buying extras for your holiday," said Which?
Which is good advice wherever you are.