Optimising code : Total on last order rows

Hello - this is how you can get the same result in less steps:

SCRIPT:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("Xc1LCsAgDEXRvWSsEPOpuhZx/9toIyktb5DJyYW3Fo3JzE2oUHtOnXZ5cSaKAEZdzUA1cv1QPNMOdsL/UM8hU8Ao6wC0qC9AP4+9bw==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Order = _t, Poste = _t, Amount = _t]),
    ChangeType = Table.TransformColumnTypes(Source,{{"Order", Int64.Type}, {"Poste", Int64.Type}, {"Amount", Int64.Type}}),
    Custom = Table.AddColumn ( ChangeType, "Custom", each 
    let 
        t = Table.SelectRows(ChangeType, (x)=> x[Order]=[Order]),
        varSum = List.Sum ( t[Amount] ),
        varMax = List.Max ( t[Poste] )
    in
    if [Poste] = varMax then varSum else null
    )
in 
    Custom

RESULT: