Can I book a flight ticket without paying immediately?

You can book your flight online at www.flycorsair.com, you can book your flight without paying immediately. 
You just need to put your reservation on hold.

To do this, click on "poser une option" (place an option) when the panel "besoin de plus de temps pour réfléchir" (need more time to think) appears after selecting your flights and filling out the personal information form.

This option costs from €15 (which may vary according to destination and/or departure date) and guarantees your fare for up to 6 days' reflection period. During this period, you can finalize your reservation at any time.

If you decide not to purchase your ticket at the end of the option period, your reservation will be automatically cancelled and no justification for abandoning your order will be required. However, the option fee is non-refundable.
 
More information on Time to think.