Not much to tell really.
You add your funds that you bought from Amazon.com and then you pre-order the game. Unlike the EU stores, you have to buy it on that day but the good thing is you're not paying £80 - £90 for the Digital Guardian Edition, instead you're only paying £52 due to the awesome dollar conversion.
Regular priced games on the US store are $60 which equals around £35. Compare this to UK PSN prices for digital games which are £54.99 or £64.99. #ROBBERY
Your EU PS+ will allow you to play online on all of your accounts.
Oh and did I mention that you can now pre-load digital games which come out on Tuesdays in the US instead of the Fridays in the UK/Europe? No reason to use EU PSN whatsoever.