I got thinking about your problem and decided to code a little something. This code will make a drop down box displaying sweatshirts, shirts, and t-shirts, as well as a submit button.
When an option is chosen it sends the user to a file called redirect.php. This file contains this code
<form action="redirect.php" method="post">
<option value="sweatshirts.html">Sweat Shirts</option>
<input type="submit" value="This One">
This file will then send them to either shirts.html , sweatshirts.html , or tshirts.html , were you can provide custom options for each type of product.
$url = $_REQUEST['url'];
if ($url = 'sweatshirts.html')
header( 'location: sweatshirts.html');
if ($url = 'shirts.html')
header( 'location: shirts.html' );
if ($url = 'tshirts.html')
header( 'location: tshirts.html' );
Hope this was helpful.