wanting to know how to easily add sums from a checkbox

im extremely new to c# so im having a little difficulty with the following problem.

below is my following code

int access = 1;
int onlineVideos = 2;
int personalTrainer = 20;
int dietConsultation = 20;

int extras = 0;

if (checkBox1.Checked)
{
    extras += access;
}
else if (checkBox2.Checked)
{
    extras += onlineVideos;
}
else if (checkBox3.Checked)
{
    extras += personalTrainer;
}
else if (checkBox4.Checked)
{
    extras += dietConsultation;
}

textBox6.Text = extras.ToString();

i want to be able to easily add the sums of all the boxes that are checked, and then print them to a textbox.