If he wants to be really clever, he could offer on-demand pricing and ensure that he doesn't get overloaded with order and fail to deliver on time. People would text and say "how much?". Then he would reply with the on-demand pricing "$5 in 20 minutes or $15 in 5 minutes."
If someone in the subway is REALLY hungry they'd probably pay $15 for a sandwich - especially if they know there's a two hour ride home.