Unfortunately, your former betta is probably going to die in a backroom now. Pet stores doesn't really have the ability to treat SBD properly. I work in a big box chain pet store, so I know. They treat with AQ salt, which encourages fluid retention. It's more of a corporate culture thing than a store by store choice. It def sounds like a mild case of SBD, which needs to be treated with Epsom salt. Something not really on hand at pet stores. In fact, AQ salt is the only treatment available for sick fish at my store (no antibiotics either, even though we sell them all).
I've just been buying the sick fish at our store and bringing them home for treatment. That way they at least have a shot. Just an FYI, that way in the future you might be more inclined to try to treat yourself.