How To Make A Simple Select Menu That Adds A Flat Rate Shipping Fee?

Can someone tell me how to make a javascript that will add a shipping price ($5.00) to any price that is listed as a value for a select menu on multiple drop downs?

<select name="shirt">
<option value="10.00">s-l</option>
<option value="12.00">xl-xxl</option>
</select>

Price: <input type="text" name="total" value="$0.00" /> (would show this without touching the drop down)
Price: <input type="text" name="total" value="$15.00" /> (would show this if selecting s-l)
Price: <input type="text" name="total" value="$17.00" /> (would show this if selecting xl-xxl)

<select name="shorts">
<option value="14.00">s-l</option>
<option value="16.00">xxl-xxl</option>
</select>

Price:
<input type="text" name="total" value="$0.00" /> (would show this without touching the drop down)
Price: <input type="text" name="total" value="$19.00" /> (would show this if selecting s-l)
Price: <input type="text" name="total" value="$21.00" /> (would show this if selecting xl-xxl)