I do not see any reason why you should not be able to purchase Apple care from anyone for the unit in question post purchase as long as you are with in said window.
I'm a Costco Member in Adelaide and at our store it's iPhone SE, 7/7+, iPad Pro, Air and Mini. then 15/13" MBP and MacBook. Air sort of rotates through as they have stock. they have had a 5K through every now and again but as i recall usually middle of the road for sizes and spec levels.
I did notice when i went to the Moorabbin Store in Melbourne (next to the airport) that their range was pared back a bit in respect to Apple Products. but they were carrying other Manufacturer PCs as well like Alienware and ASUS ROG pre-builts, at very reasonable prices considering they are of those brands.
but to answer the question posed is the membership worth it? hmm, well if your local store has a Fuel Station and you fill your tank say 3-5 times (difference can be as high as 30c/L) and you will have pretty much paid back your membership. the difference between other retailers and Costco is quite stark at times.
It's nice having Costco, extremely held back and retarded though it's Australian arm is, but once Amazon puts down roots, it's on.