“Spring for” is informal English (mostly US) meaning “pay money for”, with the nuance that the subject is paying more than what one might expect. This can be because the thing is expensive in absolute terms, or because cheaper alternatives are available, or because the subject is paying for other people (often in addition to himself).
It's common to say that one springs for a more expensive model. “He even sprung for the good stuff” means that not only did the unnamed person buy scotch for the narrator, but that person even brought good scotch, which presumably costs more. Spring for can have a connotation that the person is indulging in a luxury.
Some dictionaries (for example the Oxford Advanced American Dictionary list spring for as specifically meaning paying for someone else. But this is in fact not always the case, and I think it is not the primary meaning of the expression — spring for only connotes paying for someone else because the person is paying more than the minimum (which would be their share). For example the Camdbridge American English Dictionary gives the example phrase “To increase the radio's performance, spring for a powered antenna”: a powered antenna costs more than an ordinary antenna, but gives better performance.