Here my solution:
shipping.onSave = function(evt){
new Ajax.Updater('onepage-checkout-shipping-method-additional-load', '<?php $_urlHttp = $this->getAdditionalUrl(); $_addNew = str_replace("http://", "https://", $_urlHttp); echo $_addNew; ?>', {onSuccess: function() {
this.nextStep(evt);
}.bind(this), evalScripts:true});
}.bindAsEventListener(shipping);
billing.onSave = function(evt){
new Ajax.Updater('onepage-checkout-shipping-method-additional-load', '<?php $_urlHttp = $this->getAdditionalUrl(); $_addNew = str_replace("http://", "https://", $_urlHttp); echo $_addNew; ?>', {onSuccess: function() {
this.nextStep(evt);
}.bind(this), evalScripts:true});
}.bindAsEventListener(billing);
str_replace:
<?php $_urlHttp = $this->getAdditionalUrl(); $_addNew = str_replace("http://", "https://", $_urlHttp);