A number of online travel agents have been criticised for “ramping up prices” on bargain holiday deals.
Consumer group Which? said it had heard of a case where a customer was told she would have to pay more money or her booking would be cancelled.
Meanwhile, others find the cost of the holiday or flights increasing owing to add-ons during the booking process, such as one company charging £39 for the US visa-waiver scheme Esta, which only costs £11 direct from the US government.
Which? said passenger Kam Hughes was asked to pay a surcharge of £79 on £415 flights to Vancouver to visit her ill mother.
Ms Hughes had booked through Checknfly, which told her the day after the booking that she needed to call and confirm her flight.
Another case highlighted by Which? saw a customer stranded at the airport.
John-Michael Clow found a flight to India that was almost £200 cheaper than anywhere else from Gotogate, but he was told at the airport that his ticket was invalid.
Which? said Thomas Cook had already informed Gotogate that there was a problem with the ticket at the time of booking, but Gotogate had failed to inform the customer.
Mr Clow was given a refund and compensation after the consumer group intervened.