It may seem odd that you can "pre-buy" free bingo cards, but on some games you can. Don't worry the price of the bingo card is still free it just means you're beating the rush and making sure you're in it to win it. It is worth nothing though that some of these advanced purchase free games may require qualification - for example you might have to be a depositing player in the previous week or be a VIP player.
If you're unsure if you qualify head to the lobby of your bingo site and see if it allows you to pick up your freebies. If you can then you're in the game, if not then you'll have to qualify.