Writing to a file (Not replacing)

I have a question about an issue I am having on my final project. Within my ItemEntry.cs form, I am trying to get the application to append the already existing .txt file, rather than prompt the user to replace it. I can't seem to get the StreamWriter and FileStream to allow the user to write to the file.

Any help would be appreciated.

Code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO;

namespace InventoryApplication
{
    public partial class ItemEntry : Form
    {

        private StreamWriter fileWriter;
        private FileStream output;
        private FileStream input;
        private StreamReader fileReader;

        public ItemEntry()
        {
            InitializeComponent();
        }

        private void ItemEntry_Load(object sender, EventArgs e)
        {

         
        }

        private void SaveButton_Click(object sender, EventArgs e)
        {
            SaveFileDialog fileChooser = new SaveFileDialog();
            DialogResult result = fileChooser.ShowDialog();
            string fileName;

            fileChooser.CheckFileExists = false;

            if (result == DialogResult.Cancel)
                return;

            fileName = fileChooser.FileName;

            if (fileName == "" || fileName == null)
                MessageBox.Show("Invalid File Name", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            else
            {
                try
                {
                    output = new FileStream(fileName, FileMode.OpenOrCreate, FileAccess.Write);
                    fileWriter = new StreamWriter(output);
                }
                catch (IOException)
                {
                    MessageBox.Show("Error opening file", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                }
            }

            enterButton.Enabled = true;
            SaveButton.Enabled = false;
        }

        private void ExitButton_Click(object sender, EventArgs e)
        {
            fileWriter.Close();
            output.Close();
            this.Close();
        }

        private void enterButton_Click(object sender, EventArgs e)
        {
            Product product;
            int itemnum;
            string department;
            string supplier;
            float price;
            string description;
            itemnum = int.Parse(textBox1.Text);
            department = comboBox1.SelectedItem.ToString();
            supplier = textBox3.Text;
            price = float.Parse(textBox4.Text);
            description = textBox5.Text;
            product = new Product(itemnum, department, supplier, price, description);

            fileWriter.WriteLine(product.ItemNum + ", " + product.Department + ", " + product.Supplier + ", " + product.Price + ", " + product.Description);
        }

        private void openButton_Click(object sender, EventArgs e)
        {
            SaveFileDialog fileChooser = new SaveFileDialog();
            DialogResult result = fileChooser.ShowDialog();
            string fileName;

            fileChooser.CheckFileExists = false;

            if (result == DialogResult.Cancel)
                return;

            fileName = fileChooser.FileName;

            if (fileName == "" || fileName == null)
                MessageBox.Show("Invalid File Name", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            else
            {
                try
                {
                    output = new FileStream(fileName, FileMode.Append);
                    fileWriter = new StreamWriter(output, true);//always asking to overwrite file. how  to open existing file for appending?
                }
                catch (IOException)
                {
                    MessageBox.Show("Error opening file", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                }
            }

            enterButton.Enabled = true;
        }
    }
}

Are you sure it's not the SaveFileDialog that's asking?

Make sure you set fileChooser.OverwritePrompt to false before calling ShowDialog().