the main cart div is a form.
when you moved the promo code text box into the main cart div, you also moved the form tag, so now you have a form in a form, which not valid.
change:
<form action="" method="post" name="CouponCode" id="CouponCode">
<label for="txtPromoCode">Promo Code:</label>
<input type="text" name="txtPromoCode" id="txtPromoCode" />
<input type="submit" name="button" id="button" value="Submit" />
</form>
to:
<label for="txtPromoCode">Promo Code:</label>
<input type="text" name="txtPromoCode" id="txtPromoCode" />
<input type="submit" name="button" id="button" value="Submit" />
or move the promo code form outside the main cart div.