Due credit given to admiralvic for the idea, but if you want to get both of them, better to order the posters from Nintendo and then buy the soundtrack on eBay after they start flooding in. If there is any damage to the posters (which is more likely, especially if you wanted to buy them on eBay), let Nintendo bear that responsibility. The MM soundtrack will be cheaper to ship and barring a simple cracked jewel case (which is easily replaced), it would be safer to buy them online.
That being said, I still went for the soundtrack, as that is something I would actually use, or would be willing to sell easy enough (as I'm sure many other CAGs would).