def extra_cost(row):
    calls = row['min_1', 'min_2', 'min_3', 'min_4', 'min_5', 'min_6', 'min_7', 'min_8', 'min_9', 'min_10', 'min_11', 'min_12']
    if plan_name == 'surf':
        if calls > 500:
            return (calls - 500)
        else:
            return "NaN"
    if plane_name == 'ultimate':
        if calls > 3000:
            return (calls - 3000)
        else:
            return "NaN"
    else:
        return "NaN"

user_calls1['sum'] = user_calls1.apply(extra_cost(row))