those have to be swiped and the money loaded onto them. There is another kind with the value already assigned and just a redeem code hidden behind a silver scratch panel. Those need no processing/data addition SFAIK. You scratch the silver off, type code into Amazon site and job done (or it should be).They need to activate it thought right? I don't think the gift cards work unless they're activated at the till, though please correct me if wrong in that assumption.
Otherwise how do the £10-£100 ones work for example? I have bought some before where you can define how much is on them.