Я бы предложил примерно следующее:
import pandas as pd
df = pd.DataFrame({'customer_id': [13, 16, 13, 13, 16, 16, 13],
'Date': ['2018-01-02', '2019-03-31', '2019-03-31', '2018-01-02', '2018-01-02', '2019-04-31',
'2018-01-02'],
'Invoice_value': [920, 920, 920, 920, 921, 921, 921],
'Unit_price': [1, 2, 3, 4, 6, 7, 8]})
append_data = [df[(df['customer_id'] == ac)].sort_values(by=['Date']).iloc[-1] for ac in df.customer_id.unique()]