No need for credit card for it any more.

http://nz.playstation.com/ps3/hardware/accessories/detail/item229908/PlayStation%C2%AE-Network-Cards/

Notice that PSP games are same price as the UMD version in shops, thought download purchases would be cheaper to distribute then packaged items, and also download purchases need to be stored on expensive memory sticks.

It'll be good for stuff that's not in shops, or if know a game will get hammered guess battery will last longer between recharges on download one.