A kind of error in the code

my_list=[]
price_list=[]

def help_list():
    print("""
Info:
To review the options type: 'help'.
To view the list type: 'show'.
To save the list type: 'save'.
To end the list type: 'done'.
To load the saved list type: 'load'.
""")

def append_list(list_item,price):
    my_list.append(list_item)
    price_list.append(price)

def show_list():
    print("""
Until now you have {} things to buy:
""".format(len(my_list)))
    for item in my_list:
        print("-> "+item)

def end_list():
    print("""
 You have {} things.
 These are: """.format(len(my_list)))
    for a, b in zip(my_list,price_list):
        print("{0} -> {1}".format(a,b))
    total=sum(float(item) for item in price_list)
    print("""
Total = {} $.""".format(total))

def save_list():
    with open("to_shopp.txt","w") as file_saved:
        for a,b in zip(my_list,price_list):
            file_saved.write("{0} ->{1}\n".format(a,b))
    print("Your list is saved!")

def load_list():
    with open("to_shopp.txt") as f:
        for line in f:
            (a,b)=line.split(" ->")
            my_list.append(a)
            price_list.append(b)

def shoping_list():
    help_list()
    print("Your shopping list:: ")
    while True:
        list_item=input("-> ")    
        if list_item == "done":
            break
        elif list_item == "help":
            help_list()
            continue
        elif list_item == "show":
            show_list()
            continue
        elif list_item == "save":
            save_list()
            continue
        elif list_item == "load":
            load_list()
            continue
        while True:
            try:    
                price=float(input("Amount: "))
                break
            except ValueError:
                print("Use only numbers!")

        append_list(list_item,price)

    end_list()

shoping_list()