Having trouble making a method for sales tax in a class in python.

class Invoice:
    def __init__(self, invoice_id, customer_id, last_name, first_name, phone_number, address):
        self.invoice_id = invoice_id
        self.customer_id = customer_id
        self.last_name = last_name
        self.first_name = first_name
        self.phone_number = phone_number
        self.address = address
        self.items_with_price = {}

    def add_item(self, product, price):
        if product not in self.items_with_price:
            self.items_with_price["iPad"] = 799.99
            self.items_with_price["Surface"] = 999.99

    def sales_tax(self, price):
        sales_tax = .06
        return items_with_price * sales_tax

    def calculated_tax(self, price):
        self.calculated_tax = sales_tax * items_with_price

    def display_totals(self, price):
        print('iPad', price)
        print('Surface', price)
        sales_tax = calculated_tax(price)
        print('Tax', calculated_tax)
        print('Total', price + calculated_tax)

    def __str__(self):
        return(self.items_with_price)

# Driver code
invoice  = Invoice(1, 123, '1313 Disneyland Dr, Anaheim, CA 92802', 'Mouse', 'Minnie', '555-867-5309')
invoice.add_item({'iPad': 799.99})
invoice.add_item({'Surface': 999.99})
invoice.create_invoice()