your text field and submit button are both named txtPromoCode:
<input name="txtPromoCode" type="text" id="txtPromoCode" /></td>
<input id="couponCodebtn" name="txtPromoCode" type="button" value="" />
you must us unique names for each form element.
try changing the submit button to:
<input id="couponCodebtn" name="couponCodebtn" type="submit" value="Submit" />
see this thread:
showthread.php?t=7503
for info on applying the discount only to certain items.