I know about Netflix, but I'm the type of film buff who prefers having a physical library of tapes, disks, or whatever. As I understand pay-for-stream services like Netflix and Hulu+, and perhaps even what Amazon's doing, you're only buying a one time watch of that episode. Maybe Amazon is allowing downloads, but I don't think so. THe other problem I have with considering such services is my internet connection is not reliable. Recently my service was out for over a month, and didn't get repaired until the 10th. Since then, I have not gone 24 hours without the line dropping out several times a day, sometimes for over an hour. Suppose you lose connection in the middle of a streamed program you've already paid for?
The Netflix package I have is not a pay-per system. I pay 7.99 per month for access to all of their streamable library. It's limited - you have to upgrade if you want to receive DVDs of new material to watch, but I watch several hours a week and it's worth it to me. Also, they have a 30-day free trial, and I had recommended it in answer to the original question - original, or remastered. They have all the remastered TOS (all of the Star Trek series as a matter of fact) and for one monthly price you can watch as many as you want, whenever you want. You could check out the remastered versions and see if that's something you'd like to invest in for your physical library.
Like you, I prefer to have a copy that I can put on shelf, but this is a way to sample before you purchase. I have the original version of TOS on DVD, bought it when it first came out, but watching the remastered on Netflix has convinced me to buy the remastered version - just haven't pulled the trigger yet.