Well if it's a form, you can use $form['#redirect'] to force what page it will end up at.

To be honest I would probably, instead of using a webform that redirects to a Cart link, use a module to create a form which includes all the elements you need, and upon submission, uses uc_cart_add_item($nid) to loop through the options and add them to the cart... if that's a doable solution for you.