Two tips, from a long time AC buyer and if you're in the US. First, don't buy from eBay (too risky) and don't buy from Apple (too expensive). If you're in the US or you live in a country where they ship, head over to B&H Photo's web site - they sold me the correct AC box for $95 less than Apple's price, shipped expedited and free to my door at no extra charge. I've purchased from them, the instructions are in the box, and it takes minutes. I have also purchased AC from Amazon when the price was the same (I'm a Prime member), and Amazon threw in a free year of their UL Amazon Cloud Storage (the $60/year version, not the $5 Photo storage deal). B&H's prices are likely as good as the shady eBay dealers, and B&H is an authorized Apple reseller...
Besides, you have one calendar year from the purchase of your Mac to actually add AC to your laptop - if you just bought it, you have some time...