Nothing is charged
This site is a demonstration. The checkout asks for a card because a booking flow without one would not be much of a demonstration, but no payment is taken, no bank is contacted and no money moves. Whatever you enter, your account is untouched.
No card is kept
The number you type is used to work out two things — which scheme the card belongs to, and its last four digits — and those are all that is stored with the booking. The number itself is never written down anywhere, so the receipt shows you "ending 1234" and there is nothing else to show.
A card is accepted if it passes the checksum every real card number satisfies. That is a shape test, not an approval: it means a mistyped digit is caught before you reach the end of the form, not that anybody agreed to pay anything.
The price is fixed before the card
The fare, the taxes and the total appear before the payment step and do not change during it. If you travel with children the total already reflects what each traveller pays, so the figure by the Pay button is the figure for the whole party. See Prices and currencies for how that total is converted.
What you get afterwards
A booking reference on the confirmation page, and the booking under My bookings for as long as the browsing session lasts — which is until you close the browser, and no longer. Nothing is emailed either, so the reference really is the part you keep. See Ticket did not arrive, which is the article about what that does and does not mean.