Trate de crear el record de Net Sales, Taxes y Gross Sales como tu, pero me sale un error al previsualizar el record. Le consulte a chatgpt y me dice que es xq debo usar let ya que no puedo referenciar  dentro del record  esos campos recién creados como si fueran variables locales.

Me quedo query asi:
let
    Source = Excel.Workbook(File.Contents("D:\Usuario\Ignacio\Desktop\Business Intelligence\DATDATA Power Query\PQM 🔴 - Introducción al lenguaje M\Ecommerce Data.xlsx"), null, true),
    #"Us-Ecommerce Dataset_Sheet" = Source{[Item="Us-Ecommerce Dataset",Kind="Sheet"]}[Data],
    #"Promoted Headers" = Table.PromoteHeaders(#"Us-Ecommerce Dataset_Sheet", [PromoteAllScalars=true]),
    #"Changed Type" = Table.TransformColumnTypes(#"Promoted Headers",{{"Transaction_id", Int64.Type}, {"customer_id", Int64.Type}, {"Date", type date}, {"Time", type datetime}, {"Product", type text}, {"Gender", type text}, {"Device_Type", type text}, {"Country", type text}, {"State", type text}, {"City", type text}, {"Category", type text}, {"Customer_Login_type", type text}, {"Delivery_Type", type text}, {" Quantity ", Int64.Type}, {"Individual_Price_US$", Int64.Type}}),
    #"Renamed Columns" = Table.RenameColumns(#"Changed Type",{{"Individual_Price_US$", "Price"}}),
    #"Added Custom" = Table.AddColumn(
        #"Renamed Columns",
        "Custom",
        each
            let
                NetSales = [#" Quantity "] * [Price],
                Taxes = NetSales * 0.08,
                GrossSales = NetSales + Taxes
            in
                [
                    #"Net Sales" = NetSales,
                    Taxes = Taxes,
                    #"Gross Sales" = GrossSales
                ]
    )
in
    #"Added Custom"

A que se debe??