If I understood the whole announcement about Guild Wars 2 prepurchase correctly, to have 3 day headstart you have to buy from Ncsoft. Prepurchases from retailers will only have 1 day headstart.
The game will probably appear on steam eventually. GW1 did.
Ncsoft accepts credit cards and paypal... So if you don't like either one of those choices, some kind of prepaid visa virtual or something might work. In my case ncsoft charges in euros... Which is a bummer because I don't live in euro-zone and don't enjoy the benefits of high EU taxes(or whatever the reason they price games this high) to pay such a substantial price premium. Don't know how prepurchase will work yet but I am pessimistic. Damn you ncsoft.