Access Collection fields in Embed on Ecommerce Checkout page

The ecommerce website I manage has listings of puppies as products. Each puppy has a full price and a reservation price field.

On the Checkout page, I want to be able to calculate (full price - reservation price = pickup price) with the pickup price being a regular text element.

I’ve given text elements that are bound with the puppy’s full price and reserve price respective IDs. How can I get the values of the bound fields in an embed?

Here is my site Read-Only: LINK
(how to share your site Read-Only link)