Select option from dropdown that will link to another site

I’m designing a new WP site for a client. They have an e-commerce site already that is not related to WP. What I need to do is to link the items that are available to the new site. No problem for basic items, but with something like a t-shirt where you can select the size, I’m having some difficulty.

Right now, their (soon to be old) site is using javascript to push the size selection info to the e-commerce site. From what I can find, JS does not play well with WP. Since this is the case, does anyone know of other options?

Just to make sure I’m being clear and stating things correctly, this is what I am needing to do:

I have a static page in WP that has a t-shirt for sale. On that item, I have a dropdown box that will allow the customer to select what size of shirt they want and then they will click the ‘ORDER’ button. When the ‘ORDER’ button is clicked, the customer is directed to the e-commerce site shopping card that will have their t-shirt listed in the size that they requested.

At this time, I am not looking for a WP plugin or another e-commerce solution. I have seen some neat options in this regard out there while searching for a solution to my problem that I will have to check out sometime in the future, but not right now.

Thanks!