If you go to a big top tier public research university, it's almost assumed that it's volunteer, since there are 3948509435 people waiting behind you willing to volunteer if you decide to quit because you aren't being paid. If you go to a smaller private research university, you probably should specify volunteer. Make your email appealing. Think about what PI's want in an undergraduate researcher and what they don't want.
They DO want dedicated, hard-working, reliable people who will stay committed to their lab and positively contribute to the project and environment.
They DON'T want greedy pre-meds who are only looking for publications, don't care about the project at all, and will quit at the instant sign of a potentially better research opportunity.
Appeal to their likes and know how to emphasize that you don't have what they dislike.