I accidentally scratched off the whole code so I did the serial number thing, but it keeps saying it's invalid.. I am 500 % sure i typed it correctly. what should i do now?

If the page doesn't that you posted from help then you will need to try contacting iTunes support (you will need to give them as much of the serial number and activation code from the card as you can read) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then iTunes Cards And Codes