Creating an upsell on Squarespace

I just added a pdf to my website for sale on the store page. I want anyone who clicks on the checkout of the "pdf" product to be redirected to another store page where I have added my "Consultancy service" for sale. While this happens, I want the pdf to be already added to cart and from there the buyer can checkout with the two products.

All I want to setup is a simple upsell system for my PDF to my "Consultancy service".

I'll be glad if you can help me on this.