I don't really get why elixirs all take effect immediately upon purchase. I think it would make much more sense if the players could carry an inventory of them to activate later. There's really no reason they couldn't/shouldn't behave just like potions. It would certainly make party coordination much simpler if the members could just carry the elixirs they're planning to use rather than exit to buy them and rejoin party.
In fact, this would probably be a pretty good money-maker for the game. People who convert their plat to merchandise like elixirs are more likely to find themselves short of plat when a good deal comes along, causing them to buy more plat to take advantage of it. More plat, more money to STS. I'm actually pretty surprised this hasn't been done yet for that reason.
How it would work: elixir buff screen would show active elixir (as today) and also the other elixirs in inventory. Those that can't be activated (because another is active already) are grayed out. Player can activate one from this screen. Usual rules about when they can be used still apply.