Display arlert dialog of information when button clicked (NetBeans - JavaFX)

Question

I get stuck on display the subtotal, tax and grand total in an Alert dialog of information (displayed when Complete Order is clicked). I don't kknow how to display my calculation in alert dialog. Please advise.

public void handle(ActionEvent evtActionEvent)
    {
        //get the object that triggered the event
        Object sourceObject = evtActionEvent.getSource();
        //which object triggered the event
        if(sourceObject == quantityTextField || sourceObject == addOrderButton)
        {
            // call the validation method and if it returns a true then call purchase
            if(validation())
                purchase();
        }
        else if(sourceObject == completeOrderButton)
        {
            
            Alert alert = new Alert(Alert.AlertType.INFORMATION);
            alert.initStyle(StageStyle.UTILITY);
            alert.setTitle("Confirmation");
            alert.setHeaderText(null);
            alert.setContentText(".");
            alert.showAndWait();
            

        }
    }

 public void displayPurchase(float newSubtotalFloat, float newTaxFloat,
                                                 float newTotalFloat)
    {
        //Format the values to currency format
        DecimalFormat valueDecimalFormat = new DecimalFormat("$#0.00");
        //display the total for the customer        
        invoiceTextArea.setText("Invoice for Customer  "+'\n' +
                                "Name  " + nameTextField.getText() + '\n' +
                                "Quantity  " + '\t' + quantityTextField.getText() + '\n' +
                                "Type  " + '\t' + typeSelect()+ '\n' +
                                "Subtotal  " + valueDecimalFormat.format(newSubtotalFloat) + '\n' +
                                "Tax       " + valueDecimalFormat.format(newTaxFloat) + '\n' +
                                "Total     " + valueDecimalFormat.format(newTotalFloat) + '\n' );