Tourists face "rip-off charges" being imposed by some travel organisations, according to Which? Travel magazine.
Ultra-expensive telephone contact numbers, "hidden" extras and "free" offers that include payments were just some of the things inflicted on travellers, it added.
Some tour operators added on a £2 charity donation to the price of a holiday without giving consumers a say in the matter, the magazine said.
Which? Travel looked at the methods of 188 companies, including 51 travel companies and 32 airlines as well as hotel chains and hire companies.
It found that 28% of the organisations were using expensive 0871 or 090 calling codes; 43% were adding surcharges or unavoidable booking fees to card payments; 20% were using automatic "opt-ins" where consumers had to untick a box to avoid extra payments for things they may not have wanted; Some offered "free" car hire but with which was accompanied by an insurance package costing as much as £126.
Which? Travel acting assistant editor, Chris Gray, said: "Consumers should always have a clear choice when it comes to adding an extra item to their purchase.
"Automatic opt-ins make such choices less than clear, and we want travel companies to stop using them."
In a separate report Which? Travel said the London Olympics had sent "hotel prices soaring".
Rooms still available close to the Olympic Park in Stratford, east London, were costing up to £274 more during the Games than just before.