Why is there a price difference at the time of booking than the price advertised, or what I initially saw?

Our holidays and advertised prices start based on flight availability, fare class, travel dates, and a limited number of negotiated prices.   

These are subject to change on a first-come-first-served basis. Your holiday package price can be further affected by taxes such as airline insurance, fuel and security, other packages, tour or activity-associated costs, and currency exchange rates.