There are also different programs, including sales of Apple-Refurbished systems (which are pristine and eligible for AppleCare).
Apple Authorized Resellers make their own policies and set their own prices. For example, Amazon, Best Buy, and B & H Photo in New York sell Macs, but may not offer the generous 14-day "no-questions-asked" return policy you get when you buy direct from Apple. Or they may choose to bundle different items into their sale prices.
In my opinion, you should not consider the AppleCare PLUS program as optional, for a battery operable computer than can be moved around easily, unless you really are willing to cover all out-of-pocket expenses yourself. A broken display can cost as much as US$800 if you have to pay it all yourself.