Convert list to a string with double quotes
create_list is building a list of all the items in the string form of data. And so you can change it to:

def create_list(data):
    return list(str(data))


I find it easier to read create_command if you merge some of the lines together:

def create_command(amount):
    command = ["START"] + create_list(amount) + ["STOP"]
    return str(command).replace("\'", "\"")


And so you can merge the above two changes together:

def create_command(amount):
    command = ["START"] + list(str(amount)) + ["STOP"]
    return str(command).replace("\'", "\"")




Expanding further, rather than using str.replace you can use json.dumps to format the list. This has the benefit that it will escape ' and " characters for you, which your code doesn't correctly handle. (Thanks Mathias)

import json


def create_command(amount):
    command = ["START"] + list(str(amount)) + ["STOP"]
    return json.dumps(command)