Even though your question was already answered, my approach to be straightforward would be something like this:

>>> class Store:
...     @staticmethod # This method can’t access or modify the class state, so 'self' is not required.
...     def calc_cost(rate: float, qty: int) -> float:
...         return rate * qty
...                                                                                                                            ...
>>> def sum_stores(store1: Store, total: int) -> float: # total parameter is never used.
...     return store1.calc_cost(15,30)
...
>>> total = 0
>>> ice_cream_shop = Store()
>>> total += sum_stores(ice_cream_shop, total)
>>> total
450
>>> total += sum_stores(ice_cream_shop, total)
>>> total
900

However my recommendation at the moment to use classes would be different, you need to analyze that any state is going to be handled, so classes are good way to do this, but, this case may be simpler, we don't need a class to calculate the cost, just the alone function works pretty well and it's more readable.

>>> def calc_cost(rate: float, qty: int) -> float:
...     return rate * qty
...
>>> total = 0
>>> cost = calc_cost(15.0, 30)
>>> total += cost
>>> total
450.0

I highly recommend watch this video to avoid bad practises in OOP.