It is hard to stay afloat as a comic book store.

It depends on what I’m trying to maximize.

If I just want the happiness spike ASAP, then I would just buy $15 worth of Magic cards, or maybe a booster pack or three, depending on the set.

If I’m trying to maximize my overall happiness over any amount of time, I think spending $15 on something reusable, like a nice pair of socks or a hat would work out for me.

If I want to maximize happiness in general, then spending it on someone else I care about does double duty. It makes someone else happy, and I get happiness from being involved in the process.