adding a drop down menu for free gift on checkout or cart

hi,

hope someone can help me on a issue.

i have set up a online store using elegant themes (styleshop) and running wooCommerce plugin.

i wanted to insert a drop down menu on the cart or checkout page where my customers can choose a free product when they spend a certain amount.
is this possible, if so how will i be able to do this. at this current moment i will be getting them to add text in order note, but this does not look professional enough.

thanks my site link is below

http://orderruchi.k-hosting.co.uk/wp/checkout/

http://wordpress.org/plugins/woocommerce/