A 1G SD card obviously can't cost 1p. The postage is just to get round Amazon commission in some way. All these 1p things are sold by Amazon partners - not Amazon. I'd guess Amazon charge commission on the value of the product sold and not shipping. I know Amazon only pay their advertisers commission based on value of sold product, excluding shipping.
Doing it this way probably means the buyer actually gets a better deal even if it looks daft.