Updating fiield in PostgreSQL table using .update() not working in django - python-3.x

Question

I've been trying to update the invo_payment_method value from a table called Invoices. This is the code that I currently have to try to achieve this:
if total_payments_for_invoice.all().count() == 1:
invoice_object = Invoice.objects.get(slug=slug)
invoice_object.invo_payment_method = request.POST.get('payment_method')
invoice_object.save()
So what I'm trying to do is if it's the first payment for the invoice I want to get invoice_object which has the slug of the invoice that just got a payment and update the invo_payment_method field in the table and save the changes. But that's not working. I also tried doing the following:
if total_payments_for_invoice.all().count() == 1:
Invoice.objects.filter(slug=slug).update(
invo_payment_method=request.POST.get('payment_method'))
And it's not working either. After that I tried adding a the following to the code above:
if total_payments_for_invoice.all().count() == 1:
Invoice.objects.filter(slug=slug).update(
invo_payment_method=request.POST.get('payment_method')).save()
And this one actually updated the value in the database, but it made the webpage crash because of the following exception: AttributeError: 'int' object has no attribute 'save'. In case more information is needed, if I print the value of Invoice.objects.get(slug=slug) I get the following: INV-bb1c 21. And if I print the value of Invoice.objects.filter(slug=slug) I get the following: <QuerySet [<Invoice: INV-bb1c 21>]>. In both cases I'm getting the invoice number which is INV-bb1c and the invoice pid which is 21. Please see image attached of how the Invoice table looks like. Any help would be greatly appreciated. To clarify: invo_payment_method is initially null in the database and request.POST.get('payment_method') returns a string of the payment method used.