Problems buying facebook credits through paypal

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Recently I have been addicted to a facebook game called Tetris Battle. It is a very exciting game that you can choose to play in 2P (send lines to defeat your opponents), 4P (build 40 lines as quick as possible and be the first one) and 6P (similar to 2P) mode. Since tetris battle requires "energy" to play, I decided to buy facebook credits in order to get unlimited energy. I decided to use the money I earned from ptc sites in paypal to buy the credits. However, when I logged on to paypal, it said that I must provide my credit card details in order to continue the process. I don't want to link my credit card to my paypal account, and I don't want to use my credit card online (thats why I choose paypal as the payment method instead of credit cards!). How come there is such kind of stupid setting?? I got the same problem when I try to buy something from apple itunes store. My question is, can I simply use the money in my paypal account to buy facebook credits (or any other things), but not linking it with credit cards? What can those who don't have credit cards do? Have you ever bought facebook credits because of your favourite games on facebook? Do you find it troublesome? please share!