Select publications with "Add item", to purchase click on "PayPal" if you have a PayPal account, otherwise click on "Debit/Credit Card" in the Purchases section. Further details (name etc.) will be requested during the PayPal phase.
Publications with an ID beginning with N (e.g. N36) are A5 size, an ID of OP (e.g. OP29) are A4 size.