I did a test by putting in some dummy content in the top and bottom HTML fields in Cart Options (in product page), but I can’t figure out where they are supposed to appear. 
With the right product in the cart, I should see “Top HTML here.” and “Buttom HTML here.” somewhere on the cart page, but I don’t. Does anyone have any idea why?
This is very good question. This is a feature that is specifically for the Legacy shopping cart themes. The standard shopping cart themes do not utilize these HTML areas.
In the legacy themes, the HTML will appear at checkout at the top, and bottom of the page, when you have the product in the cart, and are on the checkout page.
Again, the standard shopping cart does not utilize these in any way, and that is why you are not seeing it.
Thank you, James, it’s good to finally know! I think this would be pretty important to mention in the documentation at http://help.infusionsoft.com/userguides/sell-online/create-products-and-services/add-a-product-in-infusionsoft as even the chat support reps I’ve spoken to didn’t know about this (they kept suggesting to check the HTML).
Hi @Sophy_Khon Good call on that. I am actually working on a lengthy project of constantly updating pieces of our helpguide. Thanks to you pointing that out, I was able to get that update made, to help out anyone else who may come across this article, while setting up.
Thanks again!