The supply items you didn’t use go away and you get refunded a small fraction of their cost. Once per battle means per fight, not per expedition.
Happy to help. And a more general piece of advice: Darkest Dungeon can be played like a game of pure number optimization and if you enjoy it like that that’s great, but it’s also perfectly viable to just go in and see what happens. I understand you’re hospitalized right now and have a lot of time to pass, but this is a long game and if you’re not careful before you know it you’ll be all better and going back to your normal life with no time to play, and then what would you do?
Seriously though, hospitals suck and I hope you get better soon, and I’m happy to do what I can, even if it’s just video game tips, to make your time there more tolerable.