at my work, we are basically honest and anyone would toss in the diffference to make up.
if I was to devise such a thing I would buy a couple of those soda slides that you put in the frighe and one soda at a time becomes availalbe.
add a simple lever with s stepper or solenoid or servo.
use a keypad, punch in yoru code, gain access. count how many you owe.
alternatly, a coin slot. available for use with an arduino.
alterntly, a wine fridge, you know half size. with solenoid on the latch so you put in money or your code and gain access.
mechancis shoudl be pretty simple if you are mechancially inclined.
there are lots of arduino soda machines on the net. you might find one that suits yours needs.