[Python] Aggregating data, and filtering on a date, in one block of code [closed]

I am trying to aggregate sales data for the summer, for each customer. So I am summing up "sales", grouping by customer - and I'd like only date > 2020-07-01

df = pd.DataFrame({
"customer": [1,1,2,2,3,3],
"sales": [56,69,32,2,45,90],
"date": ['2020-09-01','2020-08-01','2020-07-01',
'2020-08-01','2020-02-01','2020-01-01']})

df['date'] = pd.to_datetime(df['date'])

# this works successfully
new_df = df.groupby("customer")["sales"].sum().reset_index()

# this gives me an error
new_df_summer = df.groupby("customer").filter(
lambda x: x["date"] > "2020-06-01"
).groupby("customer")["sales"].sum().reset_index()


I get this error: TypeError: filter function returned a Series, but expected a scalar bool

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