By definition the billing address is the address where your card bill is sent.
It's a security feature to prevent unauthorized use of a card if it was found, if they have your card they have all the information they need except your address.
The billing address being the bank address or being written on the back of the card beats the point of this feature.

yasamoka, my guess to why your case worked is that since in Lebanon we don't have numbered addresses (buildings are not referenced by numbers) they must accept any address in Lebanon for the specific cards you're using or the bank purposely set it as its address. But note that this isn't how a billing address works. They effectively turned this "feature off".
Whatever address I try other than the address mentioned on the back of the card does NOT work. The building has a number in the address, along with a number for the PO Box, and an area name, and a valid ZIP code.

Verification fails if I try any other address.

I knew what address to use by contacting the bank itself, which told me that the address is provided there. Maybe Internet cards work differently?
I've been using different cards from different banks in the past few years and so far i never wrote the zip code or address according to the card, just random stuff, and it's been working fine for me so far. I didn't even know it was written on the back, until i read all your posts, but personally i never had an issue, so as for online, i'd say contacting the bank or master card is your best option right now.
Thank you all..I am gonna contact the bank again today and see what i can do. My other option would be to get an internet card.
7 months later
hi i am using a pic (prepaid internet card ) today to purchase cs global offensive but it didn't work, i entered my billing address, did anyone found a solution
i am getting "An unexpected error has occurred. Your purchase has not been completed." please anyone can help me?
gamingindex wrotei am getting "An unexpected error has occurred. Your purchase has not been completed." please anyone can help me?
Please don't spam across multiple topics. Check our replies on the other topic you have created and reply there or request that the other topic be closed and the replies be transferred here (if that's possible).
gamingindex wrotei am getting "An unexpected error has occurred. Your purchase has not been completed." please anyone can help me?
i think you're better off contacting on the steam forum than to contact here