I'm not exactly sure what you mean about using the available links.  If you're using 1shopping cart, then you'll probably want to stay with their links.

It sounds like you've already disabled the "buy now" and "add to cart" buttons.  Is that correct?  If not, there is a post on that in the "How to" forum section.  (I believe there's also a link to that post in the latest installation instructions in the Market download package.  — readme.html)

As a fast solution, you can place the 1shoppingcart link at the bottom of your description.

Or, you may also be able to place your 1shoppingcart link in the "Additional Details".  (You can edit the "single.php" code to change the text wording of "Additional Details" to something else — or just remove the text altogether.)

Hope this helps…