Hi I really dont know if you are planning on a local or international trip,
but for most online trips local flights to be precise, once you have a
confirmed cheap fare or any fare, the booking can only hold for 24hrs
after which it will cancel automatically.
Then for cheap International flight ticket, the booking can only hold
from between 24-72hrs of booking, you must plan ahead for your
vacation, if you are sure of your dates, then simply load your ATM
get online and issue the your ticket by paying immediately and print out live.
If you leave your booking until a week before departure, you are likely to
pay far higher than someone who buys about this time.
I hope this info helps, cheers!