Don't tell anyone else, but you should use your middle mouse button (scroll wheel click) on each poster you want. This should open the link in a new tab. Go to tab 1, click add to cart, go to tab 2, click add to cart. Your cart in tab 2 should have both prints.
And honestly you may want to try for the Gold Looper, as it seems like fewer people really want it.
As far as card or PayPal, I've lost a few by trying PayPal, so I switched to CC. I have my CC # copied to my clipboard, and just paste it in, quick change the exp date and enter my MCC code. Haven't lost any since I started doing that...