Apple Care works the same regardless of where you purchase it. With Amazon, depending on what state you live in, you can likely get it without having to pay taxes. eBay has the cheapest options, but be sure to purchase from a reputable seller, and check feedback for satisfied customers who have purchased the same item, and state that they had no trouble registering their products for extended warranties. You can save about $150 bucks overall if you go the eBay route, but either way (Amazon or eBay), you'll likely save some money.
Good luck to you, it's a worthy safeguard for what you've invested in the systems. 🙂