It's not your choice. And there is no law you can invoke that will force your employer to leave you on salary. In fact, it's more likely that he doesn't have a choice about it either.
Any employee can legally be paid on a non-exempt (hourly) basis. Not every employee can legally be paid on a salaried basis - not unless they are also paid overtime when they work it. If you're working in the warehouse that is NOT the kind of job that can legally be paid on straight salary. Unless you never work a single hour of overtime, and I do mean never, you can legally be paid hourly but you cannot legally be paid on straight salary.

