A ideia é obter o parâmetro da url e selecionar de forma automática o option quando o id for igual.
var url_string = top.location.href;
var url = new URL(url_string);
var data = url.searchParams.get(“idPlano”);
$(document).ready(function(){
$.ajax({
type: "GET",
url: "localhost:8080/RTS/WebServiceListar",
data: "",
dataType: "JSON",
success: function(data) {
var url_string = top.location.href;
var url = new URL(url_string);
var data = url.searchParams.get("idPlano");
$("#lista").append('<option class="PrecoMensal" value="'+data[0].planoId+'">'+data[0].descricao+' - '+data[0].valorMensal+'</option>');
$("#lista").append('<option class="PrecoMensal" value="'+data[1].planoId+'">'+data[1].descricao+' - '+data[1].valorMensal+'</option>');
$("#lista").append('<option class="PrecoMensal" value="'+data[2].planoId+'">'+data[2].descricao+' - '+data[2].valorMensal+'</option>');
$("#lista").append('<option class="PrecoMensal" value="'+data[3].planoId+'">'+data[3].descricao+' - '+data[3].valorMensal+'</option>');
}
});
});