Jumping in here to say I used this same process - I added my shopify buy button to my footer code injection on my site, then used the css rule provided by @tuanphan and it works great on 7.1 too! https://www.moonlightgardensupply.com/# (note, you have to have products in your cart for it to show up, but once you do, the cart appears on every page without showing the list of products I have hidden in my footer).
