Holidayextras.com has already refunded more than £1 million to customers in the wake of the volcanic ash cloud and expects that figure to increase significantly.
The site has 80,000 bookings in its system between April 12 and April 25 and estimates 200,000 customers have been affected so far.
A total of 10,000 bookings have been changed to alternative dates and the site is receiving 1,500 emails and 8,000 calls a day.
The company is working to make the booking system easier for customers to amend travel arrangements and 350 people in its head office are helping to answer the phone including chief executive Matthew Pack.
Pack has recorded a video, thanking customers for their patience.