Resuming a count value when importing a list stored with pickle as well as viewing that list with Listbox

Question

EDIT: I decided to use pickle and try that method..

The app I'm working with has a function that creates an invoice for a business. Each invoice is counted and numbered appropriately with the number of invoices created in order. This invoices are stored as dictionaries inside a list. This list is then exported with pickle to a file. Now, my problem is that I want to load that pickle file, load the list and keep saving on it following the order of invoice numbers, as well as viewing the orders already stored in the external file in the Listbox.

This would be the list the stores the dictionaries and loads the external file :

import os
import pickle  
file_path = "/Users/user1/Documents/storage/xoomdatabase.txt"
xoomDatabase = []

loadLis = open(file_path, "r")
dalis = pickle.load(loadLis)
loadLis.close()
xoomDatabase.append(dalis)
print(dalis)

This is the Listbox the displays all the invoices that are stored on the xoomDatabase list:

class AccessDatabase:
    def __init__(self, master):
        master.title("Imprint Plus Manager")
        self.master = master


        self.frame_header = ttk.Frame(self.master)
        self.frame_header.pack()

        self.frame_header.config(height = 40, width = 100)
        self.frame_header.config(relief = RIDGE)
        ttk.Label(self.frame_header, text = "Ordenes").grid()

        self.frame_verDatabase = Listbox(master, selectmode = BROWSE)
        self.frame_verDatabase.insert(END, *Database.xoomDatabase) 
        self.frame_verDatabase.pack()

        self.frame_verDatabase.config(height = 70, width = 150)

Heres the function i'm using to pickle dump:

def closeApp(self):
        with open(file_path, "wb") as fout:
            pickle.dump(Database.xoomDatabase, fout)
        self.master.destroy()

To further explain what I want: For example; I created 4 invoices today that got numbered invoice #1, invoice #2, etc. I saved the list with pickle. Opened my app again, and now i want to keep creating invoices starting from the last number of invoice counted, so it would start saving from invoice #5, invoice #6, etc. Also when I view the listbox, I would like to see all the invoices created starting from invoice #1.