Mark
Another major difference between our codes is that yours has no "return" url in the button and no "cancel" url.
Here's a copy of mine....maybe you could put that coding into the button and see if it works. (Or maybe just go back to PayPal and re-create the "add to Cart button".
<input type="hidden" name="return" value="http://www.birdhouses2luv.com/thankyou.html">
<input type="hidden" name="cancel_return" value="http://www.birdhouses2luv.com/sorry.html">
Just replace with your urls.

EDIT: I just caught you last post and FYI I altered one of my buttons with that </form></div> code and my button still worked! So probably not the culprit!