An elevator is not a swift method of transport. The door stays open for 15 seconds once you get in (assuming nobody hits the 'door open' key), takes 10 seconds to close, travels 1 floor in 10 seconds, and needs 5 seconds to open enough for a person to step through. That's 40 seconds of standing still.
40 seconds of standing still is enough for a stomach to start rumbling.
So, immediately above the elevator car, add a vending machine, whose goods drop down into a car-accessable 'catch bin'. (The sodas would probably slide to the bottom through a chute into a padded section, not just falling straight down, for obvious reasons.) You put in your coins or swipe your card, make your selection (probably from a screen), and the machine dispenses. To keep people from climbing over each other, just put two 'purchase screens' in, one on each side of the car; it's no work to have a moving slider drop the merchandise into the 'left' or the 'right' chute.
There would be no danger imposed by the added mass. The mechanism for a machine that could fit atop an elevator car only a few hundred kg, and elevators are often far overrated compared to their daily stresses. (The ones at my college are rated for 3500kg, which couldn't be exceeded without the riders carrying signifcant amounts of lead weights and solid lard.)
The sales of this vending machine will benefit by being in a high-traffic area -- and for some, an unavoidable point in their day's travel.
The riders would be able to benefit; they get munchies, and the vending machine is no longer a brightly-colored eyesore in their lobbies.