Buying Easy HTML Form Drop down - $5

I need help making what I think should be pretty easy:

I need to make a drop down menu for an online store I am making.

It needs to have four options:

• Digital Download
• Digital Download + CD
• Digital Download + Flash Drive
• Digital Download + Paper

Then their will need to be a submit button. The submit button needs to make it so the products are added to the cart.

Here comes the harder part:

The options will need to put two products together.

1. Is the actual product on the page (Digital Download) {$product.productid}

and

2. Either the CD, Flash Drive, or Paper - productid=

I tried to use something like this: <a href="cart.php?mode=add&amp;productid=17533&amp;add&amp;$productid&amp;amount=1&amp;redirect_to_cart=Y">
But I didn't have much luck.

I am using X Cart as my shopping cart, if you have any questions please don't hesitate to ask.

First person who posts something that works (on all browsers) I will PayPal the money to them.