12 x 12 Scrapbooking pages require special postage because of their dimensions, and are most often posted in 'pizza boxes'. To save on postage costs, I recommend finding a single seller or store who has the items you are after, rather than buying off a number of sellers and paying multiple lots of postage. I would then ask the seller the maximum weight that can be posted for the price quoted, and fill up the parcel with items up to that weight.
An example of this is that you might want to buy 10 12x12 pages off one seller and the postage may be a certain amount, eg $5.00. If you ask the seller, you may be able to also buy a heavier item, such as a packet of cardstock which can be included for same amount of postage, as it does not take the parcel up to the next weight catergory.
Do your research, ask the seller questions and you will be able to make the most of your purchase and postage!