This is an informational post if there's anyone out there looking for a solution for this like I was!!
Here's the situation: You have buy buttons embedded on all your product pages, which is great. You can see the cart on product pages.But as soon as you navigate away from a product page, the cart disappears.
1. Pick a random product, make a buy button for it.
2. Go to your home page.
3. Click on your menu and then edit the menu.
4. Paste the embed code for the buy button you just made.
5. Resize the button so that it's very tiny and fits in your menu.
6. Turn opacity all the way down to 0%.
AND VOILA! NOW YOU HAVE A CART ON EVERY SINGLE PAGE, INCLUDING THE HOMEPAGE, NOT JUST ON YOUR PRODUCT PAGES!!!!!!!
(Do the same for your mobile menu, btw.)