After some time reading a lot of material online and thoroughly researching on the subject, I think I was able to come to a satisfactory result.

As already discussed here (and also a related question in meta) I know that 2D or 3D packaging will be something extremely difficult (or impossible), but, as said in the question, my biggest doubt was given to the fact of how to calculate the freight for one or more products so that one of the following is not done proposals:

• sum of Frete A + Frete B;
• sum of the measures of the Produto A + Produto B and freight calculation;

Another problem found was how to add the information of several products and the webservice of The Post Office accepts only one value for each of the fields.

In this way I was able to arrive at a calculation where, so far, I believe it is returning values much closer to the actual values of the freight than with the method used previously.

Note: I did not get to officially test by calculating by code and checking the post office counter at the time of sending the product.

The logic behind the calculation is as follows:

• calculate the value in cm³ for each product (Width x Height x Length x quantity);
• add the value of cm³ and weight of all products;
• extraction of the cubic root of the sum of cm³ of all products;

For these calculations, I am using this code:

$total_peso = 0;
$total_cm_cubico = 0;

/**
 * $produto = lista de produtos no carrinho de compras. Deve possuir,
 * obrigatoriamente, os campos largura, altura, comprimento e quantidade.
 */
foreach ($produto as $row) {
    $row_peso = $row['peso'] * $row['quantidade'];
    $row_cm = ($row['altura'] * $row['largura'] * $row['comprimento']) * $row['quantidade'];

    $total_peso += $row_peso;
    $total_cm_cubico += $row_cm;
}

$raiz_cubica = round(pow($total_cm_cubico, 1/3), 2);
$resposta = array(
    'total_peso' => $total_peso,
    'raiz_cubica' => $raiz_cubica,
);

Thus, at the time of filling in the data to be sent to the post office, I fill in as follows:

// Os valores 16, 2, 11 e 0.3 são os valores mínimos determinados pelo serviço dos Correios
$comprimento =  $raiz_cubica < 16 ? 16 : $raiz_cubica;
$altura = $raiz_cubica < 2 ? 2 : $raiz_cubica;
$largura = $raiz_cubica < 11 ? 11 : $raiz_cubica;
$peso = $total_peso < 0.3 ? 0.3 : $total_peso;
$diametro = hypot($comprimento, $largura); // Calculando a hipotenusa pois minhas encomendas são retangulares

'nVlPeso'        => $peso,
'nVlComprimento' => $comprimento,
'nVlAltura'      => $altura,
'nVlLargura'     => $largura,
'nVlDiametro'    => $diametro, // não obrigatório

With this calculation I managed to achieve results where calculating the value of the individual freight of 2 products I obtained the values:

• Product A: R R 17,10
• Product B: R R 20,40

And when performing both products together I obtained R$23,60 (considering that by the measures and type of package I did not get value less than R R 17,10-it must be minimum rate).

This type of result was repeated (similarly) in several different products and different quantities, that is, there was no sum of freight but a gradual increase according to the measures and weight of the products.

Remembering that there are some outstanding features, such as validating the measures so that it does not exceed the maximum values and, if it exceeds, divide the freight into more than one"box".