there is an exeption under section 380 of the DOT code. you can fly up to four round trips a day for four days of the week. as public charters. i flew such a shuttle to St simon island GA in 2000 for Corporate Air Partners. we flew jetstreams out of PDK. flew many of Atlanta's wealthiest down to their second homes. for about 450 round trip. we flew on thursday eve, had two round trips on friday. two on sunday, one on monday. IN the same manner that 121 supplemental operates a quasi schedule such as hooters air, the now failed division of Pace Airlines, and Southeast out of SFB .
public charters have to escrow the ticket money until the flight is completed, then it is released to the operator. People sign a waiver, and are on a manifest that is checked just like all scheduled flights.
the beneftis to being a supplemental carrier is you can go into any market without much red tape unlike the scheds who must win approval for the routes or be awarded slots in the busy hub airports.
that's all I know