So, I attempted to do some research on my own (google and searching through this subreddit), and didn't quite find the answer I'm looking for.

I'm wanting to purchase Elswyr, and already have the base game. I'd also like to get my hands on Summerset, as I didn't have the time or funds for it previously while I now do. Unfortunately, when I try to purchase the option that features both upgrades, it also includes the base game and thus cannot be purchased for myself on Steam. What are my other options? I haven't been in-game for a while, so is there an option to buy the Summerset upgrade in the in-game store? I can't find a link to buy it anywhere on bethesda's site or steam. Any help at all would be appreciated, thanks guys!