Hey Alexandra, this may solve your problem even though it isn't 100% exactly what you were looking for.
What if when the customer clicks Add to Cart, the Cart pops out from the side? like so:
If that would work for you, you can get this plugin here for free: https://www.squarify.xyz/plugins/p/peekaboo-cart-plugin
quick disclaimer that I am the dev behind Squarify.xyz, a marketplace that offers premium Squarespace plugins for free, so let me know if you have any questions!